70mm needs to be stopped down, I'd say at least to f/3.2 - / f/4
50mm is really very good wide open.
Wide is very nice at f/4 - this is on a D700, very decent corner sharpness, just a hint of "dreamyness" in the corners.
Interestingly although its an f/2.6 lens, the aperture is too fine grained to be reported in the EXIF, so you never get anything cool like f/2.6 when at 28mm (although the aperture ring is marked f/2.6)