There are indeed actual 2 pc free floating handguards. They lock to the barrel nut- the Daniel Defense Omega Rail is a good example of this type, and very high quality. Midwest Industries also has one that requires chopping off the delta ring with a Dremel, but it would also free float without much trouble.
The quality differences are usually centered around the rails being in spec (the cheaper it is, the more often I hear about out of spec rails), weight (cheaper = heavier), the quality of the finish, and the profile (higher end ones have a more ovoid profile, which fits the hand better).
Quite honestly, free floating a tactical style AR isn't all that necessary. You aren't generally going to pick up much accuracy, but you will spend more. If you want to attach things, with the way the Magpul MOE is, I have a hard time recommending the less expensive (sub-$150 or so) rail systems. They aren't going to do the job any better and they'll work just fine for a forward grip and weapon light.