On my M4, I went with the 14.5" for a couple of reasons. One, it's a bit shorter, and thus I think a bit more true to military issue. And to be real honest with you, I couldn't imagine having a preban carbine and not being able to mount a bayonet, which you can't do with the 16".
As far as which FH to go with, I'd stay away from the Vortex anyway. Those tongs on them have been known to break of understress. The phantom is not only cool looking, but I've been pretty amazed at how much it reduces the muzzle flash. If you stand to the side of it at night, you'll notice almost no flash.
Hope this helps in some way.
BTW, I don't know about who has what in stock, or the prices right now. But I got my M4 14.5" with phantom and M4 handgaurds for about $240 from gunsmoke W/O bolt and carrier. Usually you can get a bolt and carrier for about $100.