air its cleaner on the marker plus you can all ways get free air on walkons
its only recently that that is correct, until not thank long ago (last few years) most sites used co2, if you needed air you had to take your own!
also not exactly true that air is cleaner, if the compressor is clean then the air is clean, it is very possible to have clean co2 and dirty air.
as already mentioned unless for a specific reason then air over co2 any day. There are some guns and some set ups i would favor co2, a sniper2 with a 50 shot hopper and 3.5oz bottle is good fun for example, but these days other than 12g sparklets ive not seen it on sites to use.