both are very fun indeed. however, when it comes to airsoft, there tends to be more cheaters because the pellets don't leave a "mark" of paint on you.
however, airsoft exists in japan, but i dunno how big it really is. i think that paintball isn't too big in japan. i remember i went to an airsoft store near akiba and it was a fairly decent size. how often they play it though, is another story.
raverboy