Madwil,

The carbon footprint etc of electricity vs NG is all very interesting and something I'm fascinated by, but it's for the China Shop.
The OP is looking for a recommendation and, as such the metered cost of the NG is likely to be far higher than the metered cost of the HP. That's what it comes down to. Will it cost $100/month or $400/month to heat the pool?

Carl