The 24-70 II, unless you get a poor one (and one hears of a very small number of disappointing copies out there - my first one had massive impact damage on the box and surprise surprise would not focus properly and was soft down one side), is absolutely amazing at 24mm.
It gets little better stopped down with respect to centre performance. Its razor sharp wide open right down to pixel peeping level and the corners are somewhat weaker but still remarkable for the aperture. It gradually improves in a very subtle incredmental way up to f5.6 when everything is perfect, but even at f2.8-4 it is better in the corners than the 24-105 L ever gets. I simply could not believe the performance when I tested mine. I have a wonderful 24 1.4 L II, but the 24-70 II has a touch more contrast and slightly better resolution around the edges/corners.
The only 24mm lens I've used that I can say definitely beats the 24-70 II is the leica-M 24 3.8 asph.
I would be amazed if the 24m IS performs better than the 24-70 II at 24mm at any aperture and I'd venture that its considerably worse in the outer field at wide apertures.