Most homes use a single system to keep the house warm and the fuel you burn will affect the efficiency of your heating. If you have two systems, pick the one that heats more of your home here.
Specifics: The type and approximate age of heating system that you have will help determine it's efficiency. A condensing boiler typically has a plastic exhaust pipe outside, and produces lots of warm (not hot) steam when working. A Combi boiler heat hot water as and when it's needed, so when you switch on a hot tap you can heat it fire up.
Heat emitter: The way the heat is transferred into your home has an impact of the efficiency on the heating system.
Gas Mains Network: If you are not on the mains gas network you qualify for some extra grants and subsidies.