Talk to Ronny (aka Jonathon-red hair tied back in a pony tail) at Michaels. Great bloke.
The best ouf of those lenses, optically is the Canon model (that's a no-brainer really...also has the largest lens hood and '
L' ring for added wank).
I dare say you'll probably get better IQ and AF out of the Tamron but the build is definatly a let down (I thought). (Size is a plus though)
Which Sigma lens did you trial? The new HSM model or the older, larger version with the Push/Pull MF toggle? I actually went though the comparisons of those lenses for Nikon a while ago (before settling on a used 35-70 f/2.8...focal length isnt as good but its almost as good as the almost spiritually awesome Nikkor 24-70 f/2.8..plus it was cheap
)
I only tried the older Sigma and found it was pretty soft, wide open...a little disappointing as if you're buying a lens to use at f/2.8, you want it to actually work at f/2.8! Hoping Also, good luck with filters for the Sigma! 82mm!! AF is pretty quick though, for the screw drive but it is louder. HSM model would probably be even better.