If your temps stay below freezing for more than 48 hrs on a regular basis, I would recommend a heat pump with backup gas/propane heat along with the usual heat strips. Normal operation would be heat pump, but would revert to propane when the outside temps drop. The emergency heat (electric heat strips) would be used only if you ran out of propane or experienced a failure of some kind with the propane furnace.
Heat pumps are very efficient if you are willing to set the thermostat and forget it. As mentioned, they take time to raise the temp, so setting the thermostat to change the temp while you are away is not usually cheaper (assuming you will be insulating the house well). You can set it lower if you will be gone for the weekend or longer, but I don't recommend setting more than 10 deg F lower than usual.
Your thermostat will need an outside air temp probe for most cost efficient operation with a 3 stage heat. This allows the unit to switch from heat pump to gas when the temp reaches a certain point. This temp is usually near the freezing point, but is actually set based on cost of gas/propane vs. electricity. With costs of propane and electricity in hand, any good HVAC installer should be able to use the formulas to determine this temp for your installation. Without an outside air probe, the unit will be capable of switching to backup heat, but usually only when the heat pump unit cannot maintain the set inside temp. This thermostat setup costs a bit more initially, but should pay for itself in reduced operating costs.
As others have mentioned, get a system with a high SEER rating. The higher, the better. The highest you can afford is my personal recommendation.
The most important thing to remember when dealing with any kind of HVAC or refrigeration is that the equipment is only as good as the installation. Find the BEST installer in your area, instead of shopping for the cheapest price. A good installer will take the time to calculate all the variables when load sizing a system for your installation, and take the time to explain everything to you.
David