AC BH online is kinda bad, takes for ever to find a game, although the singleplayer is amazing. I haven't played the 1st AC, but I loved the 2nd. Again i haven't played Crackdown 1 but the Crackdown 2 I really enjoyed. Although not as good as AC but the online for CD2 is better in my opinion.
But if you are going to purchase a game i would advise you look at some gameplay for both games aswell as possibly borrowing or renting each game and playing them before making the decision.
IMO I don't regret DL either game. Loved them both.