Opinion: Gas heater and a Chiller or Heat Pump w/cooler?

This is our first summer with a pool, and we live near St. Louis. Many pools get too hot during July, especially when we have an extended heat wave. The humidity here is also high. We didn't know if we should put in a separate gas heater (to warm quickly in the spring and fall when the days are warm and the nights are cool) and have a chiller for the hot summer month(s), or put in a heat pump with a built in cooler. I've heard heat pumps take too long to heat up your water, so you can't decide "hey, let's swim tomorrow" and have it be warm enough. I think the cost would be about the same whether we bought two units or just the combo unit. We would use natural gas. Our pool is a deep end rectangular pool, 18'x36'. Any opinions or thoughts are appreciated.