For me, security is a big issue... bikes aren't cheap, and neither are racks. Make sure the rack can lock your bikes in place -- some racks come with locks, others have them as add-ons. Similarly, make sure to get a locking hitch pin. Depending on the model, you may be able to use a standard locking hitch pin, whereas with other racks you might have to buy a proprietary one.
Alternatively, you might just get a cable lock and lock the bikes to the rack, and maybe even the rack to the hitch. Maybe I'm paranoid, but I use a cable lock AND locks that came with my rack. Better safe than sorry.
If you buy a
suspension rack, you might need a
bike adapter kit if your bike frame has a thick or oddly-shaped frame (which is becoming more and more common). A
platform rack won't have this issue. If you're comparing prices between different racks, keep the additional cost of adapter kits in mind.
Edit: When I'm hauling my bikes long distances, I like to secure them tightly to the rack with a tiedown strap. Driving down highways for hours on end can deliver a lot of vibration to the rack -- the tiedown strap is just a second line of defence in case the bike comes loose. I don't usually do this if I'm just driving to some trails in or around the city.