Oh man, once you get the hang of this it's so much fun. I just placed first with Marine, then Predator twice, then Alien. The alien is great and really fast, you stun, sidestep and tail sweep, sometimes right after you stun you get a finisher.
The higher your killstreak the crazier the finisher gets. You can still rip out a spine and skull at around 5 with Predator, or decapitate with your tail if your an alien. It's great.
The pred extra weapons rock, the razor disks, and the plasma rifle. When you get its movements fluid with cloaking etc, its unstoppable.
With Marines, the melee is really important, it auto targets anything close to you and stuns them temp. then just blast them in the face, even a pred will go down. Only use the flashlight if you really need it once you've targeted someone then turn it off right away.