Aliens do seem like a lot of fun. They sneak around in the shadows and surprise their enemies, like extra terrestrial ninja that bleed acid.
However, stealth is only good if you can pick off one enemy at a time, if they stick in a group it becomes hard to ghost people.
Predators also seem like a lot of fun. They can cloak, see in different light spectrums, and have advance technological weapons.
However, as we have observed, technology always has it's unforeseen problems.
Marines may not be able to sneak around like aliens or predators, they may not have advanced weapons or tough exoskeletons. But I would still be my money on them, cause in a pinch humans have to proved to be incredible survivors.
Over all the game looks like a lot of fun no matter who you play as.