Spike vs Angelus as in an all out, no holds barred, two enter, only one leaves scenario? We've seen, whenever Angelus is around for any length of time, that he is not really a fighter. He may have the skill, but he doesn't have the spirit. He's a schemer, a taunter, a bully, and a torturer, but, unless he's backed into a corner, with no chance of escape, he would never fight anyone who is anywhere near his equal, unless he has some sort of edge or trick up his sleeve. I don't think that anyone here would doubt that if the match up was Angelus, vs Angel, Angel would be the winner, and we saw in Season 5 of Angel's own series that Spike CAN (not necessarily always will) beat Angel. With that in mind, I would be inclined to put my money on the little guy with the bleached hair.
That might make for an interesting scenario, though. Spike somehow managing to destroy the Angelus element in Angel's persona while somehow leaving Angel intact . It would, effectively, eliminate the curse connected with his soul, since there would be no monster to revert to. We could trot the ever faithful Spuffy vs Bangel debate out again, with a whole new set of guidelines.