Paintball all the way. Airsoft does not even compare. While airsoft gives you a little pinch if you get shot, paintball makes you think twice about turning corners quick. It gives a bigger adrenaline rush and in my opinion requires more tactics.
I have a paintball team and we all have our own gear and guns. We go up to a place called Skirmish paintball about once a month to play. It has over 50 fields to play on.
Dude, have you even been hit by a 400+ FPS gun at fifty feet? Even that far away, it hurts like you can't imagine. Yes, paintball may hurt too, but they hurt in different ways. It's either (1. Get barraged with small rocks, or (2. Fifty wasp stings all over. There's this little things called density, surface area, and rigidity that make airsoft just as painful as paintball. Anyway, I have friends who do airsoft in super-realistic settings with real tactics; so don't act as if players aren't careful in airsoft. In either sport, the point is to not get hit; regardless of which one I'm playing, I'm going to be careful.
Keep in mind that I'm not saying either one is better. But they can both hurt, a lot, and in some cases, they both require the same level of caution and tactics.