I consider better to mean:
- it focuses faster
- it focuses better in low-light
- it is sharper
- it has less vignetting
I have the 35-70mm f/2.8D and am considering upgrading to the G.
I only have the 35-70mm on crop, but considering the following:
Low light performance should be the same, considering the same maximum aperture.
I would say the 24-70 is a better lens (for some values of better).
It seems like, on paper, the 24-70 is better than the 35-70.
For me, as a non-professional, the price difference is well worth the sacrifice for what still is a great lens.
You can see a good comparison between the two lenses at Ken Rockwell (Review)
His summary is
Of the sharpness of the 35-70 he says
On the other hand you should see the SlrGear.com review of 24-70 lens
The results are singularly good. This is an outstanding lens.