A very important consideration when choosing a system is the locking mechanism. The rack itself should be the first line of defense against equipment theft. Both Yakima and Thule's systems provide lock sets with matching keys. Order as many lock cylinders as needed, and they'll all work with the same lock. Rhode Gear products don't feature an integrated locking system.
Installation Finding a local dealer who can help with selection and installation is very important.
Generally speaking, the installation instructions for a well-built rack system are more than sufficient to guide all but the most technically incompetent provided the directions are followed exactly, but guidance from an expert is sometimes a lifesaver. Over the years I've installed quite a number of racks and accessories on my own, and sometimes I skim the directions. Even though the kits usually pack all the tools they need, have a good set of Allen wrenches on hand. Nothing's worse than trying to put together something with a little Ikea-esque tool. Most bike stores will help customers, as will larger sport specialty stores, like REI, that boast installation centers in many locations.