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Predator: Too goddamn easy to be seen by aliens and is still pretty hard to close on a hostile without being seen.
Aliens: The average non NPC can see the alien walking on the ceiling a rape the alien with some weapon.
Also, zerg rushing sucks, as it is the only way to fight.
Marines: Just, un-loving believeable, you will always get forgeted by alien power attacks and the detection abilities. The predator could easily rape you if you aren't aware he is there.
The only thing that keeps me playing as marines is having motion trackers.
Though the finishing moves look epic and the effects/lighting is realistic and cool.