+2. Tamron and Sigma are moving away from making alternative products that compete mostly on price - to premium quality products that equal or exceed the quality of the OEM item, while still selling at competitive prices. A really cool development IMHO.
From what I hear, I'd give the Tamron a shot instead for sure, well providing you can't manage the 24-70 II which is completely awesome. 24-70 f/4 IS might also be worth a look if you can get one for somemany hundreds below the $1500 list.