Since I left a lot hanging on the last go round, here's some elaboration from the way I see it. As said, I spend a lot of time in rental cars -- this amounts to about a dozen vehicles a year, internationally.
For a given season in a given area for a given duration, a car is going to cost a given amount. There will be no significant fluctuation depending on how much prior you prebook -- you just have to prebook instead of walking up to the counter like a dolt. Prebooking means prebooking, now days using the internet. My practice is to do it at the departure gate while I'm killing time and it takes less than five minutes. I've forgotten and had to do it on the landing roll. I've really screwed up a couple of times and done it in the rental hall. In all three cases the price was the same if I'd done it months before. Renting a car is just not that big a deal. I use kayak and stab whoever has the best deal unless there's an outfit that I really like in the area and their price is within a couple of bucks per day of the cheapest.
Which brings us to the Kemwel and AE issue.
Kemwel gets dismissed since, with all the back and forth, it takes too long to get something simple done -- they're hand-holders for people who need training wheels (or are so scared they think they need training wheels).
AE is a consolidator. They're the best of the lot (Holiday Autos is the worst and should be avoided like a social disease). But there's a problem with any consolidator -- money. (They're easy to work with and will talk to you, but it's just another bunch of hand-holding for a simple operation. Based on reports seen in these forums, their answers are often glib guesses or erroneous deductions and bear no relation to the real world.) the money problem is that you have to pay up front. If everwho they foist you off upon doesn't have what you thought you'd reserved, you've lost your leverage to step over to the next counter and get them to match the deal -- which they will. I use AE about five percent of the time if they clearly have the best deal by far, but I cringe when I do it and every once in a while kick myself in the butt for having done so.
Etalvarez brings up the question of left-handed shifting. It's a non-issue unless you start thinking about it. You can fly, shoot, or stick a finger in your nose with either hand. It's muscle memory. Leave your brain alone and your body will take care of it.
Anyway, one thing to remember is that they don't stick your name on a car six months ahead of time. You get the closest match between what they have to what you requested when you eventually show up. If you need something different it will be at the prebooked rate rather than the walk-up rate. As an example, if you reserved a small automatic and all they have is a slightly larger one, you're probably going to get that at the same price since they want to get as many cars in service as possible and don't want you wandering over to the other guy.
There may be fallacies in my thinking. I usually only travel alone or with one other person -- the smallest, cheapest car suits me just fine since I usually need to squish into some tight spots. Speciality vehicles or specific brands generally hold no interest for me. These considerations may change the entire picture.