Paintball tournament markers are restricted to 300fps... that's not much lower than airsoft. A standard paintball is somewhere in the region of 3g, about 12 times heavier than a standard .25g airsoft BB, and 15 times heavier than a .20.
A BB doesn't carry anywhere near the same amount of force as a paintball, the only reason paintballs don't do serious damage is that they're designed to burst on impact and spread their energy, anyone who's played paintball for a while will tell you the ones that hurt the most are the ones that don't burst, and they DO HURT, a lot.
I'd say they're different kinds of pain, I've been left with deep, large bruises from paintball which have taken a couple of weeks to heal but didn't really hurt *that* much at the time, whereas with airsoft I've never had a welt/bruise that's taken more than a few days to disappear.