Upload & Sell: Off
I've had both. And as others have said, it really depends on what you want. When I had the Tammy, I liked the 2.8 and the sharpness. However, I didn't like the AF and the range. So I looked into the Canon 15-85mm. It has its ups and downs, just like any other lens. Sure, it's slow to really slow when you compare to the constant 2.8. But I really loved the IS, USM, and range. I was not bothered much by things like vignetting and barrel distortion for my shooting. I really liked the wide angle side, 15mm 3.5 with IS vs 17mm 2.8 without, I took the 15mm. Of course, I made this decision when the Tamron VC was in the $600 range, now you can find it for a lot less and that adds to the trouble of your decision.
Both (3?) lenses are fantastic for the price and have fans throughout. The reason you're having such a hard time is, they're all good and you can't really go wrong (price/performance wise). You really have to decide what's better for you.
Also, I found 2.8 to not be fast enough indoors anyways, so I bought the Sigma 30mm 1.4 and love that lens as well.