There are a number of comments posted that are not quite correct, and should be clairified.
Heat pumps do not lose much efficiency below 30 or 40 degrees. In fact a modern 10 SEER unit will deliver 2 watts of heat for every watt of electricity at 17 deg F. At 47 deg F the same unit delivers 3 watts of heat for each watt in. The problem is that the air-air heat pumps lose capacity, so that an 18,000 BTU unit will only deliver 8,000 BTUs in the low teens. This loss of capacity is due to the reduced temperature difference between the evaporating freon in the outside coils and the outside air temp. This reduces the amout of heat that can be absorbed, but also the pressure demands on the compressor drops, so the power being drawn also drops. To make up for the reduced capacity, backup heat is used.
I live in NE PA and our winters can get quite cold. I have 2 zones of heat pumps in the house, which is 16 years old. Backup heat does come on, but only for relatively short cycles, and usually only when the temp is in the low teens or below.
The only other issue is defrosting. Since the outside coils are colder than the outside air, when the dewpoint of the outside air is close to the air temperature (high relative humidity), frost and ice form on the outside coils. This reduces the airflow through the outside unit and the performance of the heatpump. To get rid of the built up ice, the heatpump "defrosts" by going into air conditioning mode. This warms the outside coils, melting the ice, but also blows cold air in the house, which is warmed up by activating the backup heat during defrost. This cycle can reduce the efficiency by additional use of backup heat, but only in specific outside air conditions.
I would suggest the following: geothermal units are highly efficient due to the constant source temperature. I considered installing them myself, but the differential cost was 5 times greater. That high purchase/install cost is still true today.
On an air-air unit, look for a high SEER. but the highest available is usually not worth the price, a 13 is a good payback, but a 15 is most likely not. Also look for a unit with an intelligent defrost system. These units measure coil temperatures and defrost only when required. Lower cost units use a simple thermostat and timer, and frequently defrost when not required, and the defrost cycle lasts longer than needed. York has a very effective system on it's higher end units.
Consider installing a unit that uses your propane for backup heat. These utilize a small gas fired hot air furnace in the air stream, rather than electric heating elements. This can significantly lower the heatpump operating costs, although the initial installation price will be higher.
Have the installation done by a guy who specializes in and primarily uses heat pumps. Due to the mentioned lower duct temperatures, airflow and supply and return register locations can make all the difference in the world. If you have an existing hot air system, it may be required to relocate some registers to optimize the systems operation. My installer spent a couple of days reviewing the house design and finding the optimum register locations. As a result we do not feel air blowing on us anywhere in the house, and never feel the "cold" air that some people complain about.
I have been very satisfied with heatpump heating and cooling and highly recommend them when asked.
paul
