Do not go racing down mountains. That is how you die - esp if you are mountain biking.
Going downhill at speed takes:
1. Skill - knowing just how fast your can safely take turns, knowing how much you can brake without losing control....
2. Experience with the road - knowing what comes next and how to handle it.
Go ahead and rent and electric bike, have fun, but be careful. Going fast with out experience and skill is irresponsible and dangerous.
I don't plan on going through corners at speed. I will only do it on a straight road with no side roads. No corners and no surprise traffic.
So first I will explore the hills and when I know the road I will go downhill
No, I don't plan on going downhill at unlimited speed. I just want to experience going faster than on the flat. Ofcourse I'm going to use my brakes if the downhill is too steep.
Bike handling on descents isn’t something we humans are born knowing. A fall when climbing at 5mph hurts much less than a fall at 25mph. Ask me how I know this sometime. Please be careful.
I don't know. Just downhill at high speed. If both road and MTB trails are available then I will probably want to do them both.
Just want to know which bike is the safest for high speed descending while still being easy to get uphill with.
Keep in mind I have no experience with mountains and I also have very little experience with mountainbikes.
I think it really depends what the rental shop has in stock.
I don't think you will be able to go in with a list of specs. They might only stock a few models.
If the mountain has a well maintained road, reasonable gradient, a bit of 'fun' path. Maybe a gravel bike? If it is a mud fest, very rocky, or just solid dirt, maybe a mountain bike.
Or if you just want to get to the top of the road, a road bike?
We don't know what mountain you are going to, what path you are taking, what rental shop you are going to, so making a decision is a bit tough for us.
A road bike is safer on the road as the tyres are slick so they grip on the slick road. A MTB is safer on a MTB trail because the knobs grip into the dirt
That a road bike is faster on the road is obvious, but safer? I doubt that, the tires are very skinny and my biggest fear on a road bike is always to hit a stone or a hole or something which would cause me to fall. I feel much safer with big tires like on a mountain bike.
Its not an option because there are no hills where I live.
I also disagree with the idea you need skills to descend. I can always use the brakes to descend slower and will go as slow as necessary to keep it safe.
Ofcourse I cannot safely do 100 km/h without any experience.
But 60 should be doable for me, I sometimes do tailwind sprints where I get close to that speed.
Why would an ebike be any safer to descend with? Don't ride trails that are above your ability, an ebike just allows you to get there faster.
Ride on some green trails.
I'm just going off what you typed. You said an ebike would be safer on the downhills. They aren't. They get inexperienced people to places they shouldn't be.
I meant a mountain bike is probably the safest to descend with because it has fat tires and suspension and strong brakes. A small stone would probably not cause me to fall, but on a road bike with skinny tires I would not have that confidence.
Just to clarify – do you mean Mountain roads – like you’re going cycling in the mountains riding the roads in the Colorado or Italy or France mountains- or do you mean mountain biking? And then is descending your only concern about the bike that you rent? If you could share where you’re going and whether this is on the road or trails, that would be helpful in answering.
PS re: descending- meaning you’re not interested in climbing, more the downhill experience, or both?
Get alligator tires, or similar. Lots of thorns on mountain trails. You can also buy foam filled tires. Flats are a huge PITA
https://www.google.com/search?q=alligator+tires&rlz=1C1CHZN_enCA1101CA1101&oq=alligator+tires&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCTQ4NjNqMGoxNagCCLACAQ&sourceid=chrome&ie=UTF-8
What makes you think there are lots of thorns on all mountain trails? I've ridden mountain trails in all the states that have them in my country and none of them have thorns.
Foam filled tyres are also a horrible recommendation for proper trails as they handle like shit and have less grip. Tubeless would be way better. But either way there's not much point making tyre recommendations if they are renting the bike.
That is true, thorns vary with location/season. Foam is good for new people, but you need the harder ones - pro installed, as some people find the better foams hard to install. He could also use tuffy tire liners
Guy, you’re full of weird “facts” and advice. Last time I rented a mountain bike in an area with a goat head thorn problem. They had it running tubeless and it solved any potential issues I might have had.
OP isn’t going to install foam and Mr. Tuffy in the shop’s rental bike. And I’m not even certain he’s going to be riding trails— it sounds like he might be using an emtb on the road.
Do not go racing down mountains. That is how you die - esp if you are mountain biking. Going downhill at speed takes: 1. Skill - knowing just how fast your can safely take turns, knowing how much you can brake without losing control.... 2. Experience with the road - knowing what comes next and how to handle it. Go ahead and rent and electric bike, have fun, but be careful. Going fast with out experience and skill is irresponsible and dangerous.
I don't plan on going through corners at speed. I will only do it on a straight road with no side roads. No corners and no surprise traffic. So first I will explore the hills and when I know the road I will go downhill
A fairly experienced guy I've participated brevets with went to the mountains and crashed to death on the downhill. The risk is real.
0 experience and wanna go full gas downhill? Okay, GL my man
I get the feeling OP lacks the common sense and self-preservation most people have...
YOLO!
No, I don't plan on going downhill at unlimited speed. I just want to experience going faster than on the flat. Ofcourse I'm going to use my brakes if the downhill is too steep.
Darwin award candidate...
Bike handling on descents isn’t something we humans are born knowing. A fall when climbing at 5mph hurts much less than a fall at 25mph. Ask me how I know this sometime. Please be careful.
Exactly. He thinks just hitting the brakes will solve everything. It takes years to learn how to bike downhill at speed.
Are you looking to ride on the road, down proper MTB trails or more gravel sort of riding?
I don't know. Just downhill at high speed. If both road and MTB trails are available then I will probably want to do them both. Just want to know which bike is the safest for high speed descending while still being easy to get uphill with. Keep in mind I have no experience with mountains and I also have very little experience with mountainbikes.
I think it really depends what the rental shop has in stock. I don't think you will be able to go in with a list of specs. They might only stock a few models. If the mountain has a well maintained road, reasonable gradient, a bit of 'fun' path. Maybe a gravel bike? If it is a mud fest, very rocky, or just solid dirt, maybe a mountain bike. Or if you just want to get to the top of the road, a road bike? We don't know what mountain you are going to, what path you are taking, what rental shop you are going to, so making a decision is a bit tough for us.
A road bike is safer on the road as the tyres are slick so they grip on the slick road. A MTB is safer on a MTB trail because the knobs grip into the dirt
That a road bike is faster on the road is obvious, but safer? I doubt that, the tires are very skinny and my biggest fear on a road bike is always to hit a stone or a hole or something which would cause me to fall. I feel much safer with big tires like on a mountain bike.
You don't have the skills to descend mountains, so get some experience on small hills first.
Its not an option because there are no hills where I live. I also disagree with the idea you need skills to descend. I can always use the brakes to descend slower and will go as slow as necessary to keep it safe.
You need skill to descend fast. I think that is the overall point of the many responses here.
Ofcourse I cannot safely do 100 km/h without any experience. But 60 should be doable for me, I sometimes do tailwind sprints where I get close to that speed.
Why would an ebike be any safer to descend with? Don't ride trails that are above your ability, an ebike just allows you to get there faster. Ride on some green trails.
I'm thinking of an ebike for the uphill part, has nothing to do with descents since the motor wouldn't be assisting at high speed anyway.
I'm just going off what you typed. You said an ebike would be safer on the downhills. They aren't. They get inexperienced people to places they shouldn't be.
I meant a mountain bike is probably the safest to descend with because it has fat tires and suspension and strong brakes. A small stone would probably not cause me to fall, but on a road bike with skinny tires I would not have that confidence.
Yep. That's why they are called mountain bikes. Hopefully you stay on green trails. 
Just to clarify – do you mean Mountain roads – like you’re going cycling in the mountains riding the roads in the Colorado or Italy or France mountains- or do you mean mountain biking? And then is descending your only concern about the bike that you rent? If you could share where you’re going and whether this is on the road or trails, that would be helpful in answering. PS re: descending- meaning you’re not interested in climbing, more the downhill experience, or both?
Get alligator tires, or similar. Lots of thorns on mountain trails. You can also buy foam filled tires. Flats are a huge PITA https://www.google.com/search?q=alligator+tires&rlz=1C1CHZN_enCA1101CA1101&oq=alligator+tires&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCTQ4NjNqMGoxNagCCLACAQ&sourceid=chrome&ie=UTF-8
What makes you think there are lots of thorns on all mountain trails? I've ridden mountain trails in all the states that have them in my country and none of them have thorns. Foam filled tyres are also a horrible recommendation for proper trails as they handle like shit and have less grip. Tubeless would be way better. But either way there's not much point making tyre recommendations if they are renting the bike.
That is true, thorns vary with location/season. Foam is good for new people, but you need the harder ones - pro installed, as some people find the better foams hard to install. He could also use tuffy tire liners
rental places have foam more often to reduce flat problems
Guy, you’re full of weird “facts” and advice. Last time I rented a mountain bike in an area with a goat head thorn problem. They had it running tubeless and it solved any potential issues I might have had. OP isn’t going to install foam and Mr. Tuffy in the shop’s rental bike. And I’m not even certain he’s going to be riding trails— it sounds like he might be using an emtb on the road.
sealers might help. Rental shops want to avoid problems, so they should rent resistant tires.
None of this is OP’s problem, nor does it answer any of his questions.
true
Haha, what?
False
thorns are local. High density foams that are also high internal pressure are hard to mount by many people.
False. You don't need special tires to mountain bike.
It depends on the terrain. Earth/grass and zero thorns. get into thorn areas and after you have fixed 3 flats in a morning = different POV
Thorns aren't that common.
they are very regional.https://929thebull.com/goat-head-stickers-are-satans-spurs/