Ward would have won wide. His strength and mental toughness would be too much for Dirrell IMO. Not to mention, Ward doesn't mind winning in an ugly fashion, which I think it would sort of be.
It would be an extremely boring fight between 2 friends that respect each other. If they actually fought Ward would have figured out how to neutralize Dirrell’s speed. Dirrell always shuts down whenever the going gets tough, and he’d be even more broken once he finds out that he can’t impose himself on Ward. Ward decisions Dirrell wide in an ugly fight.