Apples and oranges IMO.
I've played paintball for years before coming to airsoft. The sports going down though due to unsportsmanly conduct/immaturity etc. It's become too competitive and over-diversified. I still have my equipment, but I gave up fast shooting speedball type setups and went pump. Pump paintball still has it's charm intact (luckily) and the community is awesome. I never really saw paintball as a mil-sim sport (I was actually somewhat against mil simmers in paintball and, to this day, I still am to an extent). In terms of force on force game quality, paintball is more fun due to the faster paced games and less "tactical" feel of things. The type of pain from getting hit from a paintball is different (hurts a wee bit more) but in some ways that fear of pain makes things more fun. Of course, thats just my humble opinion.
Basically, if you're comparing mil-sim aspects, there isn't much of a difference between the two IN GENERAL. However the age restrictions on paintball are obviously lower, and in terms of the community I've noticed more maturity on ASC. If you're comparing speedball to airsoft matches, its completely different. From what I've noticed, the percentage of people coming into paintball seeking the mil sim side of the sport over the speedball side has been steadily growing (it's now well into the majority). Speedball (the type of game I like play) is somewhat like your typical fast paced league sports like basketball/lacross etc. instead of being like a military simulation. But at this point equipment/over competitiveness etc. have begun to wear at the integrity of the sport
. Due to these little problems many older speedballers will often opt for a relaxing mil-sim game now and then to get away from buzz. (Immature kids/big kids aside) Many scenario games are actually quite well orchestrated and are very comparable both in atmosphere and intensity (but not cleanliness) to airsoft games.
Basically if I had to sum it up... if you like mil-sim, stick to airsoft. BUT if you want to try something new give paintball a shot as, unto itself (despite the few problems), it's a great sport.
Hope this helps dude