I enjoy them both for different reasons. Were I forced to choose between the two?
Alien, all the way. They're terrifying. Everything they do is terrifying. They don't need technology, are perfect in their design, and are simply unstoppable. All a predator can do is kill humans, but with a xenomorph, everyone of us is another potential Alien. There's just too many reason to list. The predators are cool looking, capable, and overall robust. But they're not even close to being as effective or terrifying as the iconic xenomorph. Pit them against each other, with no weapons for the predator, and see what happens. Biologically, the Alien comes naturally built to decimate, and to do so totally.