Pentair has a Heat Pump Calculator to determine the appropriate size Heat Pump for your location. Pentair seems to do honest calculations based on the physics and efficiencies of each type of pump and energy source.
To get the most accurate cost comparison you should change the default electric kwh, natural gas price, and propane price to actual prices in your area.
Raypak has a Gas Pool Heater Sizer. It tells you the Temperature Rise/Hr and Natural Gas and Propane costs to heat a pool in your local area to selected temperature by month. Adjust the natural gas cost and propane cost in the lower left to your local gas prices for accurate costs.