first of all, compressed air goes bad. i think that after 3 months it can not be used for breathing purposes of course.
about the compression difference, hmmm air is what 74% nitrogen, some 20 % oxy around 1%CO2 and other gases. i'd say that CO2 is much easyly compressed so you get (this is a blind shot) 10 pounds of CO2 into a gallon tank whereas you would only get 5 pound of gas mixture (air) into the gallon vessel. also, probably much less energy needed to compress the CO2.
ummm, practicly everything i wrote is from my head, figuring out why it is so, so it's probably very false, hehe.
hmm hm hm, just thought of something else, if you have 74% nitrogen, it's basicly the majority, right? so different gas has different freezeing point. blah blah, i can't explain i mean, it would take too long, but to achieve the same capacity, the nitrogen would probably only freeze your gun.
again...not sure, basicly what i want to say is, you could never achieve the same capacity.
my advice? buy airsoft guns.