As a predator, I primarily hunt aliens, rather then marines. This is simply due to the fact that if a marine sees me, I'm f**ked most of the time. I might respond quickly and nail him with a heavy and then finish him, or get him with the disc. But chances are, he sees me, he unloads lead, and unless I high-tail it out of there, I'm gone.
So I usually do a priority list of Alien - Marine - Predator. Simply because Aliens are straightforward and easy to fight, marines aren't unless they're distracted or stupid/new.
As a marine, you give up your speed for firepower. You can take out predators in seconds, you can take out aliens in seconds, you can take out -eachother- in seconds. All from a
distance.You're ment to group up, and set up a perimeter before you wander off. 99.9% of my marine kills occur when the marine never knew I was there. Any other time, well, I'm a puddle of green ooze.