I have a 2 piece drop in rail on a 16" carbine upper. Advantages to me are the fit with existing front sight post, barrel nut and delta rings assembly. I live in New Jersey and the pinned muzzle break makes removing any part of the upper problematic.
Most true free floats require that you remove the gas block, barrel nut and existing delta rings and front guard cap and attach the hand guard directly to the receiver or to a specially threaded barrel nut. In terms of attaching accessories like MBUS, vertical grips or lights there is no real difference. The only con is that you do not get the accuracy improvements seen with a free float and the 2 piece drop in may not feel as rock solid as a guard anchored to the receiver. Minor tradeoff in my opinion. The AR-15 is not designed with free float handguards as a must have and is plenty accurate as is for me and the types of shooting I do.
With each type, 2 piece or free float, just make sure the gear is up to the task...bottom dollar e-bay "bargains" may be more trouble than they are worth, they will never be tight enough, won't align properly to the receiver rail and may get so hot that you cannot comfortably hold them after several rounds. Yankee Hill, Midwest Industries, Troy, LaRue and Daniel Defense makes great products at different price points. Also check out RRA's website, they have a lot of different configurations and their pricing is not too bad.
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Last edited by Jpyle; 12-13-2012 at 05:52 AM.