Have an ancient heat pump (plate with model specs is barely legible, pretty beat up, probably quite inefficient) and considering replacement. Saw some discussion in another thread about rationale for NG/electric - quick heat of spa vs. maintain temp with a cover, cost of therms v. kWh. If I recall correctly, NG is good choice for quick heating of spa (likely what Iwould do most of the time: in Miami so never really cold, and that's my 20-month old's private 1K gallon little pool), but previous owner apparently did "heat" entire 15k gallon pool (not to broth temperature, but something comfortable) at least once or twice per winter.
Also, don't recall which thread I saw the formula to compare therm/KwH. For me: 10.56Â¢/kWH and $1.15/therm.
Supposedly, cost of similar capacity gas/electric unit are similar. Additional cost of switching to NG would be running the gas line.