The best way to answer this is to meet with a qualified HVAC installer. Your outdoor design temperature is 18 degrees. This is well within the range of a stand air source heat pump.
To determine which is better, the HVAC contractor should first do a Manual J calculation to determine what size unit you need.
- Your current unit is probably oversized by a ton or so.
- This means you are wasting money and losing energy.
See the article here to see how to check the size of your current unit.
Once the Manual J is completed, the HVAC installer can do energy modeling for you, or recommend an energy rater who can do it.
- The results of the energy model will predict the cost and savings between the heat pump and the RCC.
- My guess is the you will get more benefit if you take the additional money that a RCC costs and put it toward sealing your duct work.
One last tip, see if your local electric utility has any incentive programs, they may pay for the energy modeling!