I think the Predators would win because they have no problem with self sacrifice in the name on honor, they're not afraid of death, while the Engineers feared whatever was chasing them and didn't have any traditional weapons. Predators would have ran at the threat and blown themselves up if they were losing. I stand by my theory that the Engineers are scientists and used humanity as lab rats. They fight with dirty weapons, not one on one combat. They may fear combat, which is why they use bioweapons. Predators are a bit crazy and that's hard to beat in a battle.