Propane is much less sustainable than electrical power because there are finite amounts of fossil fuels. Electrical power is almost limitless with solar, wind and water power. Nuclear reactors are also in play as nearly limitless heat sources for turbines. When and if humans can control fusion reactions, that type of power will be much more sustainable and viable than propane's limited capability.
I think humans can constantly find new ways to generate renewable electricity while propane is a finite amount that must be extracted from the earth, over time I think we will continue to find new ways to generate large amounts of electricity at lower and lower cost and the world will benefit.
I think that electrical power is more sustainable than propane gas. Propane gas, while being a good source of energy, is attainable, it just isn't as efficient as electrical power. And the ways to attain Propane gas is limited while there are various ways to attain electrical power through different tactics.
If you include solar and wind power under electric because they usually charge a battery, I would go with electric. The power that is converted from solar and electric usually needs to go through a sine wave modified inverter to your house,but works very efficiently. Propane is separated from oil and natural gas,its in limited supply.
I feel that electrical power is more sustainable than propane gas. Electrical power is used to power vehicles and office buildings in a clean matter that doesn't destroy our earth. Using gas such as propane is ravaging our earths resources and eventually this will run out. Electric power is a much cleaner solution.