Xenomorphs would be fucked because I can't imagine they could tear apart a decently built terminator. But they could melt them with acid and can have massive numbers. Predators can use their weapons to tear them apart and have actual combat training I'd imagine but there wouldn't be enough of them to win straight fights. Terminator's have number advantages but are probably weak in single combat against the Predators.
I like your thinking! I just made this thread to see people's theories. I agree with you on the predators, not enough of them to win in an all out war. I think it would be either xenomorphs or terminators that'll be on top.