I've not seen a review in Popular Photography re the new Canon 24-70 f/2.8 II or the newer Canon 24-70 f/4.0. There is an oblique reference to the Canon f/2.8 II in the August 2012 testing review of the new Tamron 24–70mm f/2.8 Di VC USD stating that the Tamron is not as sharp as the Canon 24-70 f/2.8 II at 70mm, but there is no supporting data for the Canon, only the Tamron. What I'd love to see is a comparison of the testing data on the the original Canon 24-70 f/2.8 versus the new Canon 24-70 f/2.8 II versus the new Canon 24-70 f/4.0 versus the new Tamron 24-70 f/2.8 IS. Has anyone seen any factual comparisons of the new lenses, or know if Popular Photography plans to publish test results anytime soon?
I haven't looked for any side-by-side comparisons, but if you google tamron 24 - 70 review, you'll get several sites to look at. I suspect that a side-by-side comparison would show the Canon better at some focal lengths and f-stops and the Tamron better at other focal lengths and f-stops. Practically speaking, I don't think you need to worry too much over this choice, in that both lenses are going to yield good results. Mainly, I think the biggest consideration is price; if can afford the Canon, go for it. I agree with Skipper, who has said in the past on these pages that she likes buying the camera manufacturer's lenses because, in her view, they just seem to work better together. I have to agree, but I've been known to buy a third party lens when price was a big considration.
While our official 24-70 II review isn't official yet, I can see that the need for VC is probably your biggest consideration in making this choice. The Canon is going to be sharper on the whole than the Tamron as a matter of course. It's also built a lot tougher (in my experience with it). It just depends on how much you need that VC.
I recently bought a used Nikon 24-85mm, f3.5-4.5, lens without vibration reduction (VR). Although Nikon recently added VR to this lens, I mostly use the wide end of the zoom range where I don't miss VR that much. So, I saved a few bucks and bought the non-VR version. I'm sure I'll eventually encounter a situation where I wished I had VR, but, hey, when you're trying to save a few bucks here and there, VR is not, at least in my case, a high priority.
I'm in the weird scenario where I have a 24-105 F/4L, but I'd also like one of the new 24-70s (even though it's way out of my budget at the moment). The IS is really great for shooting hand-held stills, but the 24-70 just feels like a better lens for when I'm shooting weddings or events. It all depends on the kind of shooting you do, I guess.