I have both the 24-105/f4 IS and the 24-70/f2.8 (not the version II). I think they are both great lenses. I did some testing, taking pictures with both (5D mk II) and found it difficult to see any differences. The IQ of the 24-70 is just a bit better (mostly contrast). The main differences to me is where one lens does something the other can't:
- The 24-105 can do 71-105 that the 24-70 can't.
- The IS and lighter weight of the 24-105 makes it a nicer all around walk around lens.
- If I need f2.8 for DOF or moving subjects then the 24-70.
The new 24-70 II sounds like a killer lens, but the price is prohibitive for me - more money than both the lenses I have without the flexibility. Might get the version II some day, but happy for now.
For portrait work on a tripod, the IS is not needed, for hand held it is great. I think for portrait work I would go with the 24-70 or even better the 85/f1.8 or best the 85/f1.2L (on a FF body).
Jim 'Carnac' Carnahan