All backpacker canister fuels are liquefied petroleum gas (LPG). The key difference between them are their vapor pressures as their stored energy is relatively similar. The article posted by Jaxz has a very detailed explanation about this.
Vapour pressure indicates the evaporation rate of the gas (liquid to gas) at a specified temperature. If you have ever shaken a fuel canister, you would have noticed that it contains liquid, the liquid evaporates and supplies the burner with gas. The rate at which the gas is supplied to the burner relates to the vapour pressure of the gas at the operating temperature.
At temperatures above approx 10Deg the vapour pressure is not a concern and all gas mixtures will have very similar performance. At extremely cold temperatures there is a substantial performance reduction of some gas mixtures.
- N-Butane (Normal Butane)
Has the lowest vapour pressure of all gas canister mixtures. At temperatures below 0deg it stays a liquid and does not evaporate. Hence there is no gas that can be supplied to the burner. At temperatures just above freezing the evaporation rate (supply) will be far less than that required by the stove (demand) and the stove will sputter.
- Propane
Highest vapour pressure of camping gas mixtures. Only a liquid below -42Deg and will therefore have better performance at lower temperatures. BUT it needs a thicker heavier canister for storage and therefore it is not used in its pure form for these lightweight canisters.
- Isobutane
Preferred canister fuel. Liquid below -12Deg. Lower canister pressure than Propane and therefore lighter weaker canisters can be used.
Now it should be noted that the boiling temperatures I discussed above are at sea level (0m ASL). However elevation does not influence the performance of gas canisters, with higher elevations comes lower temperatures and therefore reduced performance. Hence the fact that higher elevation influences canister performance due to a drop in atmospheric pressure is a myth.
So now to compare brands:
- MSR isopro – (80% Isobutane/20% Propane)
- Jetboil – (Isobutane/Propane)
- Eiger (75% N-Butane/25% Propane)
- Cadac (N-butane/Propane)
Isobutane is more expensive that N-butane. Hence the price difference between MSR and eiger for instance. But extreme cold temperatures will influence the performance of the cheaper brands. Therefore for colder temperatures I would advise using fuel with Isobutane rather than N-butane.
As Riaang noted putting a canister on a stand will improve the performance there-off. Another neat little trick is to immerse the canister in water.Gas canisters cool down during use due to some interesting thermodynamic principles. This cooling and the ambient temperature therefore influences the performance. If you immerse the canister in luke warm water during cooking, heat exchange will take place and the canister will remain at a higher temp.
Also buying a stove with a pressure regulator will increase the performance in colder conditions regardless of the fuel mixture.
I use a MSR Windburner with MSR Isopro fuel and I can confidently say that there exist no stove (including JetBoil) on the market that will beat this combo in efficiency and boiling time at sub-zero conditions in the Drakensberg.