Where in the Norhteast? Lots of people around here use regular air heat pumps. I have solar and do others in my area (NY, just north of the city). Don't know of anyone with a ground source heat pump.

Gas heat is cheapest to install, most expensive to run. We visited many neighbors with pools before putting ours in. Since we don't have piped natural gas, lots of them had propane gas heaters. They all said that it was so expensive to run that they only heated the pool for company! Not what I wanted!