No offense to those who own Tamron lenses, as I've owned a couple of them (28-75 and 17-50 non VC) myself in the past... They always seem to have a yellowish hue to them, most subjects kinda looked like they had hepatitis
I tested the Tamron 24-70mm VC at a booth at Photoworld Asia awhile back and though I saw richer colors, the yellowish tint was still there for the portraits I took. By testing, I mean exposing properly with AWB, custom white balance, and kelvin. There's just not much of a way around it when in camera, but there is (for the most part) in post. So, if you shoot a lot of people like I do, and want minimal post, I'd avoid third party. Not saying they're bad for everyone, just bad for me.
My vote goes to the older Canon 24-70 because the colors that came from it were quite good. I owned the Canon 24-70 for a long time and didn't see too many people complain about it until the Mark II came out. =P The 24-105 I've never owned but I'm sure it has it's own specific place in the world.