Fixed or flip, Troy is the way to go. Period.
I have a set of Troy BUIS on my M&P15T that came with the gun. I also have a set of GG&G BUIS on one of my Armlites. The difference between the two is pretty obvious when you look close at them side by side, and shooting proves it out (both guns shoot pretty much the same with optics on them).
The Troy's dont lock up solid and have a lot of play in them. They work OK for what they are, but if thats all youre planning on using, Id look for something else.