I have the GM II version, as when several years ago I was first making choices of Sony lenses, although the 24-70mm is one of my favorite zoom lens ranges, reviews seemed rather mixed on both the first generation of the 24-70mm GM and the 70-200mm GM. Since there were rumors about new versions coming along, I decided to wait, and my patience was eventually rewarded. I very much like the 24-70mm GM II -- it's a comfortable size and of course very versatile. I like that it is close-focusing, too, so I sometimes use it in lieu of a macro lens when I want to get up close and personal with a subject.
The 24-70mm GM II is indeed somewhat smaller and lighter than the first generation one. The lighter weight is definitely more apparent in the 70-200mm GM II as compared to its predecessor, and I have really appreciated this weight reduction in both lenses. To me it's worth the extra money!