I have both of the FE standard zooms (more as a side effect of a bonus bundle than by choice) and they're both good lenses.

The 28-70 is definitely much better value though, and it's the lens I grab when small, lightweight and dusty are the key words...

To answer your question: the wide end (28) is better than the long end (70) but the central area is pretty consistently sharp from wide open. The corners improve up to about f5.6 or f8 at the long end. There are lots of reviews and measurements about and I seem to recall that the DXO measurements seemed to match my observations pretty well.