I need to replace my 15mm fisheye - I owned the Canon 15mm and was very pleased with it (albeit on a cropped sensor camera - D60 then 20D at the time) but it was stolen with some other gear. I've replaced other lenses, but I wasn't sure about this one - alot of you seem to be shooting with the Sigma. Is it a better lens, or is the difference mainly economical? I'm shooting with a 5D now, so edge sharpness is very important. Thanks.

Extreme corners are not perfectly sharp on either. Canon looks like it may be marginly better in the corners, but the difference in performace is not as great as the cost difference.

Other difference is that the sigma focuses closer.