Just to add a quick note to this, CO2 is a bit temperature fussy as poil27 mentioned - you tend to need to space your shots as opposed to shooting like a maniac and shooting in winter gets... well, cold... er... HPA in winter, CO2 in summer is a good bet.
I would suggest that you look into small CO2 tanks like 9 or 4 oz since a 4oz will give you about 10x the capacity of a 12g co2 - plus, it takes more shots to drop the temperature the same degree.
I would also suggest a buddy bottle if you use HPA... Being able to top up a rifle without lugging around a pump or a scuba tank has advantages.
Oh... and fill SLOW.
Cheers!