120-400 or 150-500

[Nighthawke]

Presuming you mean the current versions with OS they are as good as each other. Plenty of good reviews around. The choice is really whether you want the benefit of the slightly wider end of 120 or longer reach of 500mm. Both are fairly heavy but I find the 150-500's weight helps steady it. Google search for reviews but don't pay too much attention to comparisons with Canon L glass because the Sigmas cost a lot less and not in the same league. That said many report the quality as very close on a sharp copy.