For several reasons I'd prefer another Predator before another Alien(s).
We've had more Aliens movie and there isn't much more to be done with them in my opinion. We've seen one alien, several, a queen, a predalien, a weird humanish lookin alien. It's been done alot and the only way I can see the concept being made fresh is to maybe explore the space jockeys more or visit the alien planet and try to kill em or an origin story of some sort.
We've only seen two Pred movies and there's a lot more dynamic. Predators are all so different plus we can explore their hunting culture, their technology ... to me, there's more interesting possiblilities with Predator.
Plus I'm a huge Predator fanboy and I need more Predator movies to watch about now, thanks.