CO2 = easily removable from vehicle, requires no electrical/wiring, quick to fill a tire. CONS: needs to be refilled ($$$), tanks can be large/bulky, can be expensive
On Board Air = constant air flow, can* be economical, small/compact (easily hidden), light-weight. CONS: gotta run electrical, can be slow to refill tires, can also be expensive, takes toll on battery
that being said, i'd rather have a nice air compressor/on board air any day over a CO2 setup. i don't really feel like needing to pay to refill a tank or run the risk of running out. A nice fuse box setup, a yellow top optima (or similar) battery, and a proper wiring job is the only real requirement for OBA. also have to choose your mounting point wisely. I've seen folks run it under the hood, under seats, in rear cargo areas, rear quarter panels, in bumpers. With CO2 tanks, folks were stuck with them in back seats, rear cargo areas, or in the beds of vehicles.