Well, you need to do some math to figure out which is the most efficient and cost effective... lots of people are saying heat pump, but a heat pump doesn't work when it get's below about 40F outside *I THINK* and then you're back to dumping amps through the heat strip.
Roughly, I think gas/propane is about 3 times more efficient than a heat strip, and a heat pump is about 5 times as efficient as a heat strip, so, I might be wrong but the way I see it, you would need to be below 40 degrees more than 50% of the time you need heating, for gas to be more efficient than a heat pump / heat strip combo.
base it on 1,000 hours
(AC/Heat pump and heat strip)
500 hours at 5 times efficiency of heat strip(heat pump) = 2,500(units of heat, just an example) + 500 hours of 1 times the efficiency of heat strip (heat strip) = 3000 units of heat per dollar
or
(AC only and Gas)
1,000 hours at 3 times the efficiency of heat strip (gas / propane) = 3,000 units of heat per dollar
or
(AC/Heat Pump and Gas)
500 hours at 5 times the efficiency of heat strip(heat pump) = 2,500 and 500 hours at 3 times the efficiency of heat strip (gas) = 1,500 for a GRAND TOTAL= 4,000 units of heat per dollar
Having both a heat pump AND a gas furnace would be the most efficient, but I seriously doubt it would be worth the cost of install and refilling of the propane, it just depends how cold your winters are? There's a lot more math to go into figuring just how many hours each winter you're under 40 degrees, how much it would cost to heat with electric during those hours verusus how much it would cost to heat with propane during those hours, and then figuring out how many years it would take you to see the same amount of savings in energy cost to cover the cost of the propane install. If it's more than 8-10 years it's probably not worth it.
EDIT: actually, I'm not even sure iif there is such a thing as a heat pump / gas furnace combo (I'm an electrician, not an HVAC guy! lol) but I don't see why there wouldn't be, other than that the cost of such a system would far outweigh energy savings you might see. Any HVAC guy care to clear this up?