Gas heaters are the most powerful heaters on the market. They can rapidly heat your swimming pool to high temperatures. If you are looking to heat your pool through the cold weather or on short notice, then gas heaters are a must have. If used irresponsibly, gas heaters can be very expensive to run. Poor water chemistry can destroy gas heaters quickly (not covered under warranties), so it is important with this type of heating system to pay close attention to your pools water chemistry.
Heat pumps are more expensive than gas heaters, but are more economical to run. Heat pumps cannot rapidly heat a pool and are not as powerful as gas heaters. These heaters are great if you are interested in maintaining your pool at a reasonable constant temperature. Similar to gas heaters, poor water chemistry can damage your heat pump.