I have another question... what are the advantages of having a free-float rail as opposed to regular non-free float? I understand if I wanted supreme accuracy I would want to eliminate any forces on the barrel including those from my hand and jerking the gun around... but does it really make that big of a difference?
I've kept a shooting log for awhile now and one thing I noticed after adding a Knight's free-float to my 16" Bushy HBAR was that my average groups tightened up about a half-inch at 100yds. The only other change I made at that same time was adding a Magpul M93 stock, so I am inclined to credit the free-float tube.
A gentleman going by the handle "Molon" at AR15.com did a similar before and after experiment that was much better detailed and thought out than my log book; but he arrived at the same result, about 0.5 MOA improvement.
The other edge to the free float is that because it attaches directly to the barrel nut, the better designed ones act as a big heat sink and help cool the rifle better than the non-free float tubes. However, it is a small enough difference that I don't know there would be any practical effect to it.
If you are looking at rails, I believe Bravo Company is selling the Daniel Defense Omega rail for $179 right now. No tools needed to install it and lighter than most rails.
Having said that, I'd probably go with the MOE, Magpul's new angled foregrip and a Surefire G2 in a cheap mount for my next build (assuming Magpul comes out with a midlength MOE).