Hi Coulee;
Have you ever owned either type of rack on a previous car?
Personal preference has much to do with it, but there are a few things to consider if you haven't formed that preference.
For a rear mount, some models simply strap onto the trunk lid, with another piece that sits on the bumper. These are cheap, fairly easy to install/remove (after you've done it once and set straps and knobs where they need to be for your car), and easy to store when not in use. They are also about the cheapest option out there. However, they totally restrict access to the trunk, and are not the most secure - the rack can move around a bit, just through vibration, and that could cause damage to the finish of the car.
Other rear mounts take advantage of a trailer hitch - this has the benefit of maintaining full access to the trunk, and is *very* easy to mount/unmount from the car. Of course, there is the added expense and hassle of the trailer hitch itself.
The other comment on rear racks is that they are subjected to substantially more abuse than bikes on a roof rack - driving on a muddy gravel road is a big no-no unless you can wash the bike before riding!
The biggest benefit of the rear carriers is the ease of loading - no lifting bikes above your head, no need to remove wheels, and easy access to locking mechanisms. If you are not comfortable lifting bikes over your head onto a roof rack, this is your only option.
For a roof rack, there are also a couple of options - fork mount (where the front wheel is removed from the bicycle), and upright (where the whole bike is attached to the rack). I prefer fork mount - they are more rigid and secure, require slightly less acrobatics to get in place, and also stay out of the wind a bit more. The benefit of the upright is, obviously, that you do not have to remove and separately transport the front wheel.
Roof racks are a bit trickier to install/uninstall until you've done it a few times, and can be a bit awkward to handle when not in use. The other concern is remembering that it is there - I have witnessed, on a couple of occasions, what happens when a low beam entrance meets bikes on a roof rack. With a rear mount, every time you check your mirrors, you are reminded. The roof rack also adds additional flexibility, be it for transporting other gear (a travel box, kayak, skis, etc.), or just bringing a small load of lumber home.
You'd have to do some comparison shopping, but on the whole, a roof rack is probably going to be, at best, about the same cost as a trailer hitch/rear mount setup, and then up from there.
Anyway... I think that's enough from me... hope that helps.
