Took the words right out of my mouth!!
CO2 doesn't effect the trigger but does effect the ratchet because the A5 uses surplus gas to turn the ratchet and load the marker - I've played in cold weather with CO2 and it just won't work! Air all the way but buy a high output preset and a 4500/5000psi 1.5l bottle - An A5 won't re-cock on low pressure, especially with extended shooting and because the A5 is gas hungry having a small bottle just sees you refilling a lot, which is fine for walk ons but if you intend to play big games the last thing you want to do is run out of air at an inopportune moment.