From what I have heard of this lens the sigma is indeed the better performer - however often people quote it as being a heavy lens as well as make comments regarding its larger front element in comparison to the nikon and canon offerings.
However in all honesty the weight is not that unusable and the only problem with the larger front element is that it requires a larger filter size.
The only real problem is the quality control issuse - something that sigma does sadly appear to have a problem with. Personally I would say send the lens back under warrenty and have it refocused if you get a soft copy. The ideal is to send your camera along and have the whole setup calibrated (that is lens calibrated perfectly for your camera body) however that means no camera for some time

Too bad JerryPH is no longer here, he was a big proponent of the Sigma 50mm f/1.4. I think he still answers PMs, maybe ask him about it. I've heard it's great also, better than the Nikon/Canon, but it's like $500 and much bigger like Mike said. It takes a 77mm filter. My Nikon 50mm f/1.4 AF-D takes a 52mm!
If I didn't already own the Nikon, I'd seriously consider it.
Sigma AF 50mm f/1.4 EX DG HSM (Canon) - Review / Test Report