Effect of Atmospheric pressure on Butane /propane

[stupot]

I am trying to establish the effect on the efficiency of butane against propane gas, when used from standard containers, of reduction of atmospheric pressure. Practical observations by different users show differing opinions about the effect of use in high altitudes (markedly lower atmospheric pressure) and the differences between butane and propane in that situation. Temperature has an effect on the boiling point of the gas as dose the pressure of the gas within the container but how does low atmospheric pressure alter the efficiency of the gas as a means, for example, of heating, if at all?

Thanks!