I would avoid the Sigma 50mm f/1.4. A high percent of those lenses have major AF issues [youtube.com]
. This is NOT a firmware issue and cannot be corrected with reprogramming by Sigma.
I have one and it's a lottery really getting a good copy, got my perfect copy after 3rd replacement. PITA but it's worth the hassle as this just an amazing lens, it's on par with L lenses IMHO.