I own the 24-105 and used to own the 28-70 (previous model to the 24-70). I ended up selling the 28-70 because:
- the 24-105 is just as sharp
- I can live with the barrel distortion at 24mm on the 24-105 as I have a 24 TSE for architecture
- 24-105 is a much more versatile reach, and for my kind of shooting the f/4 is more than mitigated by IS
- the 24-105 is quite a bit smaller and lighter
However, it does focus poorly in low light (e,g at an indoor event with moving people)
The 28-70 is a great lens and I'm sure the newer 24-70 is too, but unless you need the f/2.8 (for DOF or low-light focusing) then get the 24-105.