The 24-105 is tempting, I could use the extra reach frequently and don't really care about the 2.8 of the GM 24-70 (still need to use it once in a while). But as much as I can praise the reliability and image quality of the GM I'm afraid, and therefor have not purchased, the 24-105 is a bit more like the 24-70 f4, soft flared when backlit and overall just not as sharp. The GM version is actually on par or better then some fixed focal length from other manufacturers, be that good or bad. So I guess subject and photographic filed could make that decision. Controlled environment with little backlight and shooting portrait, the 24-105 comes out on top, landscape and general photography, no matter the light, it's the GM 24-70 (and maybe a little cropping).