Heh, I have the same question right now.
I'm undecided between Canon 24-105 L f/4 IS, Tamron 24-70 f/2.8 IS and Canon 24-70 f/2.8 L II.
The Canon 24-70 f/2.8 L II clearly has the advantage of resolution wide open but it lacks IS and is much more expensive. I'm not sure if this lens is worth it and if the lack of IS isn't worse than having a little less resolution with the Tamron. But then one can compare the Tamron with the Canon 24-105. They have comparable resolution and contrast. Which of these would you take and what would you shoot with it? Do you need f/2.8 or would you use f/5.6 or higher anyways?
Also between the 6D and 5D III, clearly the 5D III has the better autofocus but if one only uses the center point that doesn't matter. Then the 6D should have slightly better image quality. But it has only one card slot, so a no-go for many professionals. I'd like to take the 5D III because I'd like to experiment with that autofocus and maybe later I can switch back or upgrade to the 6D II in a few years.
So many options, so many opinions...