Propane is a cleaner fuel than oil, but oil is not combustible in air and is safer. Other factors to consider include cost of fuel, the existing heating system and cost of replacing the furnace.
When comparing the two fuels themselves, both are more expensive than natural gas, as of 2015. Both propane and heating oil prices have a history of rapid changes making it difficult to predict which offers the least expensive option. In some regions, availability of the particular fuel is the determining factor.
For many homeowners, the least expensive option is replacing the existing unit with one that uses the same fuel. Both propane and heating oil require storage tanks. Changing to another type of system adds the cost of replacing the tank and plumbing to the installation.
The cost of the heating system is often more dependant on its efficiency than the fuel type. Electric furnaces are the least expensive to install, but more expensive to operate than either propane or heating oil systems. The Annualized Fuel Utilization Efficiency percentage allows consumers to compare efficiency between systems. The higher the number, the more efficient the system and the fewer emissions it creates. In colder climates, higher AFUE units provide long-term benefits in cost of operation.