A sharp lens is only good if your body nails focus. If you do any shooting in fading or low light, or sports or other moving subjects, the 5D3 combo hands down. If you're shooting with strobes in a studio or landscapes on a tripod then clearly the 5D2 combo is king. The fact is no one can give you any kind of worthwhile advice without knowing what, where, and when you shoot.
Don't necessarily limit yourself to f2.8 glass either. The 24-105 f4 can be a killer piece of glass with 3 stops of IS and when combined with a 5D3 body.... about 4 stops higher ISO (that translates into a LOT of action stopping shutter speed) compared to what you're use to.
The IS has absolutely no benefit for subject movement. It only helps to correct camera movement allowing one to shoot at lower shutter speeds. It provides no 'action stopping' advantage. The camera itself does not provide 4 stops improvement in high ISO performance over the MkII.
Agreed. I did not say anything contrary to that. The 24-105 f4 DOES provide 3 stops of IS that the 24-70 does not (FWIW). AND the 4 stop advantage in ISO I mentioned is in relation to his existing T1i camera. And that's 4 stops of ISO that can translate into very good shutter speeds that will easily stop action.
For the OP's benefit... I came from a 7D with 17-55 f2.8 IS to the 5D3 with 24-105 f4 IS, and frankly, I don't want the 24-70 f2.8. It doesn't help hand holding in dark venues like cathedrals and other similar dimly lit attractions that don't allow tripods. And when you're in those kinds of situations, you seldom want the narrow DOF that comes with f2.8 anyway. And, I can achieve a very similar DOF shooting portraits at 105mm @ f4 that I use to be able to achieve with the 7D at 55mm @ f2.8. And that extra ISO the 5D3 provides over my old crop ensures I always have a decent action stopping shutter speed for shooting people in bars or restaurants. If you really want a narrow DOF for artistic shots and subject isolation, then even f2.8 is not usually good enough at wide angles so you need a fast prime for that kind of work. Compliment your 24-105 with a fast 50mm or 35mm (whatever suits your preferred field of view). The other benefit to the 24-105 is the weight... it's a lot less tiring after walking around all day in a new city. Let's face it, the 24-70 is a studio lens. It's meant for work on a tripod or in studio lighting or both. It's not an ideal walk around lens.
Last but not least, I cannot emphasize the importance of a good focus system enough. The 7D was no slouch on focus, but my keeper rate in dimly lit situations went from about 50% to 95% when I upgraded to the 5D3. The 5D2 is not going to offer you much improvement in this regard, if any. I strongly encourage you to buy the most advanced focus system your budget can allow... and then start putting good glass in front of it. A shot that's in focus from a cheap lens is much more useful than a shot that OOF from an expensive lens.