Depends on your area what is most popular and least expensive. Natural gas is usually run in the cities, propane in rural areas. Gas dryers are more expensive than electric if that's a concern at all. Many parks are all electric as they don't want propane tanks at every pad, but some have gas/propane. In my area, gas is cheaper than propane and propane is cheaper than running electric. Electric heat is cleaner and safer. Look to what is customary in the area and the costs, that's what will tell you what's in demand by tenants.