You will generally see two different types of hitch mounted bike racks. Ones that carry the bike across the top tube and those that carry the bike using a wheel tray. Here are some pros and cons.
Bike racks that carry bike across the top tube pros are less expensive than tray systems, can lift to get into trunk and can get the bike onto the rack fairly easily. Cons are some racks allow bikes to sway and damage each other, bikes have to be removed one at a time to get to the first bike, some bikes have no top tube and you will have to buy attachments for these and most of these racks typically only come with a cable lock.
Bike racks with a tray system improve on access to individual bikes, prevent swaying damage and some have better locking systems they are typically more expensive though.
You can go to etrailer.com to browse some options.
Last edited by ratdog; 07-06-12 at 06:02 PM.