How much lighter? I'm the fence between those two lens. I'd like the speed of the 24-70 and I don't need anything past 70mm (I have other lens) so I was thinking of that faster lens. Now I hear the 24-105 is lighter (key for me) and I'm thinking, what the heck, get the extra focal lenght and forget the speed and weight.
The 24-70 weighs 2.1 lb vs the 24-105's 1.5 lb.
The 24-70 is 4.9" long vs the 24-105's 4.2"
So the 24-105 is a little over a half pound lighter, and around 3/4" shorter. And, there is the image stabilizer which is a joy.
BTW, I absolutely love the image on my 28-70 2.8L, and 2.8 is great. But for walk-around, it gets to be a bit much. I'm looking forward to a tiny extension on the wide side, and a big extension on the tele end.