There are a number of things to consider when picking a new pool heating system. Even something as simple as sizing the right unit is complicated by a number of factors - for example aboveground swimming pool heaters need to have quite a bit of power.
In fact above ground swimming pool heaters need to be more powerful than the heater that will heat a similar sized pool that is dug into the ground. The reason is simple - insulation. The ground provides excellent insulation (which is why a house built underground is so environmentally friendly). If the pool has walls exposed to the air then it loses a lot of heat. One thing you should do with an above ground installation is insulate it - it will make a difference to your bills.
The other thing you will need to consider when buying your new gas, oil of electric swimming pool heaters is the location of the pool. The other big heat loss factor is the wind - any wind whips the air from the top of the pool, meaning the heater has to work harder. This is why a smart investment may be some kind of windbreak - a fence or some bushes for instance. Place them so they do not cast shadows over your pool, though, or you will lose out on the direct solar heating from the sun!