I am trying to decide between these two with the new 5D3. I have used a 17-55/2.8 Is on my 40D for years and have been waiting for the 5D3 to upgrade. I know the capabilities of the 24-105 and am wondering if going for the 24-70 without IS on a camera (5D3) that can obviously tolerate higher ISO would be the way to go.
I like the reach of the 24-105 and I dig the IS. I am often shooting at 1/15 of a second and the IS is a huge help in those situations, although that is on my 40D and I never go above ISO800 and even then I try to avoid it.
Any thoughts on this?
Neither the 24-70 nor the 24-105 is going to be a great solution for shooting in very low light but the 24-70 would probably do better. The 24-105 is a a stop slower than the 17-55, so that would increase shutter time to 1/8s, which is VERY slow, and your keeper rate would be worse than with your current 17-55.
But FF cameras tend to have less noise than crop bodies. Assuming that the 5DIII's ISO 3200 performance is equivalent to the D40's ISO 800 (worst case), that would give you a respectable shutter speed of 1/60s for the 24-70 f/2.8 and 1/30s for the 24-105 f/4 with a couple stops of IS thrown in. So for static subjects, the 24-105 might do better and give you more flexibility with ISO, but I think I'd rather take my chances with the 24-70 at 1/60s because faster shutter speeds also reduce motion blur.
That said, you might consider getting the less expensive 24-105 (esp. in a kit) as a walkabout lens and then use the several hundred dollar difference for the 24-70 I or the over 1k difference for the 24-70 II to get a f/1.8 or f/1.4 or f/1.2 prime at the focal length you need. An f/1.4 lens would allow you to shoot at 1/60s at ISO 800 whereas the f/2.8 lens needed 1/15s. The DOF is different, but fast primes on a FF camera would give you the most options.