The type of free-float tube that you want depends on the type of shooting you do.  If you shoot DCM-style matches, you must get a tube that goes inside the handguards and does not change the profile of the rifle.  These tubes are only for 20" bbls, and very strong if made/bought from a reputable source.  The other type of tube that replaces the handguards altogether I have no first-hand experience with.