Bohica 30 or 36" standard upper.
I have the 24" and although it is light enough for me to shoulder-fire unsupported, it's not easy, and not practical at all. So IMHO you might as well get that extra velocity for long range shooting, and fire it from a supported position. I have pushed my 24" beyond 1000yd and it does OK, but I tend to think that a longer tube would not only give me a 200-400fps boost, but would also cause more consistant burning of the slow .50bmg powder. Even so, my 24" shoots sub-MOA with Amax's, H50bmg, and new CBC brass.
All that said, the Bohica is the only .50bmg upper that I've shot, so I don't have experience with other brands, but for what I want to do with it (long range shooting/plinking and pyrotechnic bullet launching) it does better than I expected it to. Customer service with the Bohica in my experience has been great, and it's the cheapest option out there.