I think the pred mask is to prevent the aliens from getting into their throats (like that poor pred dummy in AVP1) and laying a chest-buster. There would be no pred-aliens if he only kept it on.
Anyway... Preds all the way. The aliens are cool but there's something about a gun/knife/acid/camo/laser-happy alien race that makes me all warm inside. They even have alien shuriken (HOLY SPACE NINJA, BATMAN).
My FAVOURITE reason though, the ultimate in badmind: Even if they're losing, NUKE de place FTW. NEVER DIE ALONE, DAGNABBIT!!!