I posted
this mini review a few days ago regarding my impressions of the Tamron vs Canon. Both are very good lenses, but if I had to choose one it definately be the Tamron. The Tamron has that extra stop advantage and is sharper than the Canon at equivelent apertures. It's VC is as good as Canon's IS (the Tamron is also quieter), but the Canon does seem to focus slightly faster and has the extra 35mm. Both are weather sealed and the build and size/weight are also pretty equal.
For me the Tamron is the more usable and better lens, but many people cannot see past the fact it is not a Canon L. Coming from F/2.8 on a 7D, I felt thet the "kit 24-105 lens" was holding my 6D back. The 24-70 F/2.8 just feels right and fits my needs better. It offers >95% of Canon 24-70L II IQ with >100% of it's features (IS is not offered on the Canon) for half of the price. The Tamron is a winner.