I really think the 24-105 f/4 IS L is an under appreciated and under valued lens. Maybe not in this thread, but overall.
I am wondering, do you really find the 24-105 that that much more discrete on a 5DIII than your 24-70? Very good lens, no doubt, but comparing 24-70 I vs 24-105, the 24-70I is 0.1" wider and 0.6" longer. The 24-70II is 0.2" longer and 0.2" wider.
I almost invariably use lens hood when I'm shooting, and I agree though the lens is only 1.5 inches shorter with hood on and almost the same when extended. The hood shape of the 24-70 just seems to make it so noticeable (despite being quite functional.
5D mark III, 50D, 17-40 f4L, 24-70 f2.8L, 70-200 f4L IS, 28 f1.8, 50 f1.8, 85 f1.8, 100 f2.8 Macro