I went with the Knight's Armament RAS II system. It cost $275 for the SHOT Show special package deal, which included the free floated rail system, 3 11 rib (full length) panels, 1 6 rib panel, and a Knight's vertical grip. So far it seems rock solid and won't ever droop or move. The great thing about it is that you can install it without any special tools and without removing your front sight base. It's an excellent rail system if you ever plan on using an Aimpoint with it. Both LaRue and Knight's Armament make low profile mounts for use with the Aimpoint M2 in conjunction with the RAS II. Personally I'm going to get a LaRue mount in a few weeks when I buy mine. Or if you prefer, you can still use an ACOG scope.
Note, just because this system is free floated doesn't mean you can't use it. All you have to do is remove your old hanguards and clamp the top piece onto your barrel nut and flat top rail. Then you slide in the bottom portion and clamp it to your barrel nut too. This doesn't require any special tools or know how. All you need is basic mechanical ability, the provided allen wrench, and a flat head screw driver to tighten the flattop rail clamp. The benefit of a free floated tube is generally a slight increase in accuracy since nothing is touching/pulling on the barrel itself.
Here's my rifle: