I'll agree with CDS on this one. I recently went through this purchase decision, and ended up with the Tamron 24-70 VC and it is excellent. The VC is very good, the IQ is a vast improvement from my copy of the Canon EF 24-70 Mk I it replaced, and the USD is nearly as quick and just as silent as Canon's USM. I doubt I'll ever notice a real world difference in focus speed during use.
For me, I wanted f2.8. That means the f4's were all out of contention. I already have a 24-105L IS that came with my 5DIII, which will probably be sold now that I know the Tamron is so good.
I wanted IS for low light shooting such as weddings and birthday parties, where I don't necessarily need super fast shutter speeds. That was the main drawback I saw with the 24-70 II.
The kicker for me was that Tamron doubled it's rebate recently to $200, and CPW had a street price $100 off at the same time. So I was out the door, brand new with 6 year warranty for $999.
Price wasn't necessarily a big factor, since I would have most likely bought the 24-70 II as a refurb from Canon whenever it got back in stock, since I had the 24-105 to hold me over until then. In the end though, the benefit of having VC, a longer warranty, very close IQ vs the 24-70L II, similar AF performance, and saving money on top of that meant that the Tamron was the best choice for my bag in the end.