If you want to get the maximum use of your pool but minimize the heating costs you might look at using more than one type of heater.
Perhaps the most obvious option here would be to have a solar heating system backed up by a gas heater or heat pump. This would mean that you could take advantage of free solar power when it is available but still have a means of heating the pool when the weather is poor, early morning/late evening and outside the summer months.
As well as keeping heating costs as low as possible this would also be an environmentally friendly option but of course it has the disadvantage of being considerably more costly to set-up so is not a viable option for many people. However it is worthwhile looking at you much energy you could generate using a solar heating system to see if it makes financial sense. Have a look at this site to see how you might do that.