If you are asking about brands, I have only owned Hayward and AquaComfort. I think they were about equivalent in quality. If you are asking about size, I would buy the one with the highest BTU rating that you can afford. As proavia mentions, it comes down to how long you want to wait for the pool to heat. Heat pumps are great for maintaining the temperature of a pool and not so great at getting to temperature. In the spring, it could take several days to get to swimming temperature. I would suggest solar covers are a requirement if you have a heat pump. On cool May or September days, I could lose enough heat if I left the cover off at night so that it would take most of the day to get back to swimming temperature.