Alamo/National do the "choose your own car" thing nationwide, it is not Orlando specific. With Avis, you will get a car assigned. As always, you can ask for something else. You can also scout the lot beforehand and suggest the car you would like to have, they might give it to you, or not.

Note that there are two separate rental areas, one in the "A" side garage, one in the "B" side, about a 5-7 minute hike away from each other. In a pinch, if you don't see something you want, you can go across the terminal to the other garage and check there.