They would probably be disinclined to fight at all, and if for some reason they were forced into gladiatorial combat by Ming the Merciless I think they would team up, bust out the arena and kill him instead.
Seriously, um... Probably Sarah Connor. Ripley has had to fight for her life too but I think Sarah got more of a chance to train (at least after the first movie). And yeah, she's crazier, which in this case would probably help.