Hello,
I can't speak for the canon, but I own the sigma and find it excellent. With my 40d it resolves LOTS of detail and I often print to A1 size. I have posted several photos recently using this lens if you care to have a look. The price for the older (slower) lens is now very competitive too. Can't see why you would need this lens to be fast in most applications, I nearly always use it stopped down.
If you would like a high res image emailed just let me know.
Hope this helps.
Dave