Azumah Nelson is the superior boxer. But, Nelson's prime was at Featherweight. He made a good account of himself at lightweight against Sweet Pea in a losing effort, but, really did not accomplish anything at that weight. Mosley was a big lightweight. It would be a bridge too far for "The Professor", even though I am likely to rate him higher than Shane historically.
Mosley is 5 9 and was simply massive at lightweight. Nelson is way better but too small and gets smashed to pieces in about 4 rounds.
Moseley by easy decision Nelson was no lightweight. He did give Whitaker a decent fight, but primarily was at 130 lbs and 126 lbs.