R8 but it needs to have V10 , R8 with V8 is slow
Not from what I see. The V8 looks to be low 4's to 60 and mid - high 12's in the quarter mile whereas the V10 is mid-3's to 60 and high 11's in the quarter. There's a difference no doubt but I wouldn't call the V8 a slouch by any respect.
Also, you have to keep in mind that while this might matter to you, it won't matter to anyone renting the car. They're looking to drive an R8, not an R8 V10 specifically and most won't even know there's two different versions of the car. The guys renting Lamborghinis at the exotic car rental places out here aren't advertising motors because they know people don't care. They just want to drive a Lamborghini.
Also, it needs to make financial sense. For me, if it can pay for itself and I have access to it when it's not being rented, that's all that matters. And I personally don't care at all about the difference in power. As it is, my daily driver would rock them both. So if it were just about straight line speed I wouldn't bother with either.
do the rental places help you figure out which is in more demand? and how abusively people drive these things?
Nah. You basically just have to research what's out there, how many trips a car has relative to it's asking price, look at it's schedule, etc. That's why I'm really conservative with my math because there's a ton of unknowns.