I personally think geothermal is great, sadly New England's geography makes getting a ground loop installed more difficult. In the midwest lots of people would do a horizontal loop by trenching like 8 feet underground laying pipe in there. In many parts of new england that isn't really viable with all the rock so vertical drilling is used which is significantly more expensive.
Our house was built in 1973 and originally had electric baseboards and then was converted to an oil boiler with hot water baseboard. So retrofitting a heat pump was an interesting task.
A ground-source heat pump typically has a CoP of 5 but the CoP doesn't go down with the outside air temperature. Because the ground water temperature is stable, it will maintain its efficiency when it is cold outside. Of course, depending on the R value of your house, it may need to work harder to make up for the heat loss through the walls, roof and windows.
You really need to find a heat pump installer who has the year round temperature data for your area and can do the math to calculate whether it is worthwhile to spend the extra on a ground-source heat pump.
Typically it makes the most sense to do when you need to replace your system.
