The 24mm 1.8 afs is sharper than the 24mm 2.8 afd (which I sold several years ago). As long as I can remember, the 2.8 afd suffered more from color fringing.
Having said that, the 1.8 is currently not my favorite lens (actually I hesitate to sell it again): the images look very harsh to me, somewhat unpleasant, a little 'overcorrected'.