A big decider in what gun you should get decides on how/where you play. If you play indoors, you'll want a more compact gun that allows you to move through tight hallways easily. If you play outside, you're going to want a gun that shoots at a higher velocity and has a longer barrel. The M4/M16 series is ideal for outdoor but some still use them indoors.
I know a couple guys that bought a GBB pistol as their first airsoft gun since they play lots of indoors. And they get just as many, if not more, kills as the guy using full rifles (me).