The other question that comes to mind is are you planning to use your pool everyday or leave it for a week or more at a time. For everyday use you'll probably want the heat pump since it's supposed to be far less expensive to run on an everyday basis. But if you have to heat the pool up because it has sat, then gas may be the way to go.

Have you considered solar? You said you have a great deal of sunlight. Trust me, it works where you live. I give this as an example:

My parents had an 18x33 oval 4' deep. I installed a 4x20 and 4x10 panel for them--just the two panels. Their pool (and the panels) only got 6 hours of direct sun a day (too many trees), yet the pool which NEVER went over 80 in years past, stayed at 84 all summer.

The kicker: their house is in Ossining. That's Northern Westchester.