The advantages of a freefloat rail system are that they remove extra pressure input on the barrel, and allow for mounting accessories. This is mostly useful for precision shooting.
If a light and grip are all that are to be mounted a non-freefloating rail system would work just as well. If sights are to be mounted a free float system may be prefered.
LaRue is known for making top of the line stuff for AR platfrom rifles. Other respected manufacturers are Daniel Defense, Troy, and Knights Manufacturing. Others who are functional but do not seem to have the same following are MI, and YHM.