I also happen to be a happy owner of the 28-300L as well as the 100-400L.
The dust problem is very little to none as far as I can tell from the amount of spots I can find when enhancing contrast to eliminate them in PS
The image quality of both is excellent.
I tend to use the 28-300 from 50 mm and above. At 28mm it is better to stop down a bit to lets say F/8 - F/11 to boost sharpness a bit.
From 50-300mm its sharpness is great! From 28-50 it performs like the normal grade wide-angle EF
Also the IS
on this lens is much much better than on the 100-400.
Just check my pics in the DB
. A lot of them were shot with the 28-300.