The debate as seen from a field owners perspective.
In paintball I have somewhere in the neighborhood of 20 yr's experience and with airsoft about 1 yr worth. So take from my opinion what you may.
Paintball in it's inception was started first as woodsball.... a bunch of guys having at each other in what ever camo they could find playing capture the flag. As the sport evolved the challenges changed and speedball was born as well as the development of scenario games for woodsball players. Speedball became more mainstream taking the heat off of the whole shooting people debate by getting sponsors and wearing flashy colors and changing the over all appearance of the paintball gun then referred to as a marker. While the woodsball players got more into the camo and mission orientated side of the game at their local fields. With the development of the NPPL speedball grew in leaps and bounds almost dwarfing woodsball altogether. But non the less woodsball always had its place and survived the onset of speedball. As the speedball genre began to taper off and the publication of many paintball magazines the cheating or "wiping" as its most commonly called became more and more apparent as the prizes got bigger and bigger. With scenario games now picking up speed and the amount of players attending the events increasing at an alarming rate the birth of milisim gear came out. With the development of the milisim gear more and more scenario games became more military orientated and so did the markers which at this point now start to look more and more realistic. To date woodsball is now back on the rise and since the NPPL recently declared bankruptcy the whole speedball arena is now in question. More and more companies are producing military styled markers with numerous accessories and body kits to change the markers overall appearance.
The main differences between the two sports is that airsoft uses a plastic BB and tends to be more hard core military style play as well as the guns them selves are very accurate representations of their real life counter parts. Aside from the gear used the only real difference is the style of play, ammunition used, guns used and their operation.
Paintball is typically more expensive to play on a regular basis simply due to the cost of paint. With players spending on average $25-$35 for entry and $80-$120 plus per case of 2000 rounds it gets expensive really fast. The guns pricing range is similar to those of airsoft from cheap to insane expensive so there is no real difference there when it comes to that part. Airsoft players tend to spend a bit more on admission pricing due to the fact they use their own ammo. Using their own ammo is a benefit as it is cheaper per 2000 rounds then their paintball counterparts. When a player can spend $10 for some quality BBs in the same quantity as an paintball player the cost savings becomes evident. Beyond these points there are very few differences between the two sports other than game play.
My overall opinion is that it is far cheaper to play airsoft on a regular basis then it is to play paintball.