I had the cheapest Bell for years and recently upgraded to the most expensive Thule (when it was on sale at REI) for the security features. If you never leave your bike unattended, a Bell can be satisfactory. Some people will be bothered more by its flimsiness and wobble, though we felt it was stable enough. We wanted to be able leave the bikes unattended on the car for short stops, such as while eating at a restaurant. The Thule has steel cables instead of nylon straps, a key lock for the cable tightening ratchets, and another lock for a cable that passes over the outermost bicycle top tube. It also weighs a lot, so it doesn't have the slightest bit of wobble.
A few months ago I switched from my old Allen rack to the Thule with added security features. Well worth the money. More stable at 60 to 65 on the highway and more secure against theft.