I tested both and ended up keeping the Tamron. For me it was the economically smarter choice . I loved the fast AF on the Canon, and the Tamron is noticeably slower there, but not so slow that I'm missing a lot of shots.
The VC works well, I've heard folks say it's not needed, but I like it. Took the Canon to the circus and it was pretty dark in there, so a slower shutter and less ISO would have been nice.
I haven't needed to do any MFA with the Tamron, might do some more testing on that, but so far so good.
I've had a few shots with pretty obvious onion bokeh, but it didn't bother me enough to pick the Canon over it. Overall, I think it has very nice bokeh.
I'm happy with the sharpness of the Tamron, comparing shots with the Canon I don't see a huge difference, but I also haven't compared corner sharpness, where I hear the Tamron is lacking. It's sharpest at 24mm, but I think even at 70mm it's pretty good.
I've had some onion bokeh appear, it didn't bother me too much though. I think overall the Tamron's bokeh is pleasing though. Never had onion bokeh in the Canon, I think micro-contrast is a bit better in the Canon, too.
I think the Tamron is a good buy when a rebate is going on and using greentoe.com for an extra bit of savings. I've seen them used go for <$900, so I'm thinking that will stay as the used price. As PP mentioned, Canon's used pricing is all over the place, I've seen some go for 1800 and some for 1600. That will probably regulate once Canon's pricing scheme is fully in effect. I was all set to get the Canon, but then reconsidered because I can't justify spending that much when I have other lenses I use more anyway.
Both the Canon and Tamron are outstanding lenses IMO, and I'd happily own either one!