Which is more efficient, CO2 or HPA?

I am new to paintball, and put huge emphasis on air efficiency and the yield I can get from a tank of propellant. I have heard mixed answers about the efficiency of these two gases, so with an equivalent volume of gas, which one will fire more paint? Also, if I use anti-siphon, is CO2 acceptable in electropneumatic markers? And will I always have to hold my gun perfectly upright with an anti-siphon device installed? Thanks!