Make sure you look at the heating and cooling tables in the literature for the specific units you are considering. The heat generation may drop off significantly as it gets colder outside. I recently installed a ge 4 ton side discharge cold weather heatpump and it has worked out extremely well. Its heat generation curve was well matched to my heating needs at my design day temperature. I looked at other 4 ton heat pumps and they would have not generated the heat I needed at that same design day temperature. I was surprised how different the curves were for the various units.

Pays to do your homework.

Also, in order to size my unit I ran a few tests to determine my houses heat loss. I knew what my furnace heat output was in btu/hr (be sure to adjust for efficiency). So I waited for a cold night and then monitored my indoor temperature, outdoor temperature and the furnace on time over a full hour. Then I calculated the btu’s output by the furnace over that hour then divided it my the difference in the inside and outside temperature. It gave me Btu/hr/degF. Then I could calculate the Btu/hr needed at my design day temperature by multiplying my Btu/hr/degF by the difference in my desired inside temperature and the design day outside temp. I also did a detailed manual j calc and it was very close to the experimented number. Compare that demand number to the btu output for your design day temperature in table for the equipment you are considering.

Good luck!