I think in no way you will be wrong if you buy the 10-22 from Canon, it's quite a capable lens offering the 16-35 range on FF. I had a short experience with it and I loved it. The only thing that I see people get third party SWA lenses is either because of $ or because some of them offers faster aperture like a F2.8. However, if you think that a F3.5~4.5 is good enough for you, I could suggest the Canon without a doubt.


IMO, i'll stay clear with Sigma zooms as they never come to satisfy me (although the new 24-70/2.8 seems promising.), the focusing problem is a concern as well. Some of their recent prime lenses seem to be good as I once owned the 150/2.8 Macro; I also used the Sigma 50/1.4 and it's a nice lens overall.


(sorry, got the very first sentence wrong, have edited)