Xenomorphs are by far the better alien. They are so much more horrifying and... alien.
Predators are cool and all, but they rely on guns and technology like us, realistically, we humans are just as capable as they are. Didn't Arnold kill them all single handedly?
But the aliens are raw animalistic danger. They don't kill you, they bring you near death, and then rape you with their disgusting toothy tongues and impregnate you. And after you think you've escaped... well, you know what hell happens than.
For me, i think Alien is the cooler of the two movies. thats just my opinion. i've never seen AVP but i'm assuming that if predator wins, the director left out that Alien had acidic blood and grew to its size in a day or so in the first movie.