Hi Caleb; In terms of cost to operate, electric pool heat pumps cost only about 2/3 less to run compared to gas heaters. However, heat pumps won't work in freezing conditions, when the outside temps are below 50 degrees F.
Gas heaters work in any weather condition, but used during winter can be expensive, especially if you don't use a pool cover. It depends where you live, as to how expensive it will be to use, but I'd plan on $100-$200 during warm months, and $300-$500 during cold months.
Sizing of a pool heater is not super critical, but larger heaters are slightly more efficient, and faster to heat. Heaters are sized by their BTU output. Most inground pools need a 250K or 335K size, or the 400K size, if you have an attached spa or large pool. Pool heat pumps (electric), for pools run in the 100-150K BTU range.
Pool heat pumps are about twice the cost to buy, as compared to gas pool heaters, however, the cost to run a gas line may be more than the cost of an electrician to wire up a heat pump. Depends on where your house gas meter is located, and how far of a run it is to the equipment pad. Let me know if you have other questions. Rob