All Teams are balanced to each other from what the developers are saying, you just really need to practice with each Race to get there full potential. I also played a bit of AVP2 a week or 2 ago and found that no one tries sneaking around as the Alien and just rans and attacks, I got so many silent kills by just waiting around dark areas, if you say camping its far from it considering Predators can see you no matter what and Marines can detect you if you're moving.
I'm going to enjoy playing Marines on AVP3, I mostly want to do co-op stuff on them though :D, I'm so sick of I WANT TO BE A COOL PREDATOR.