Off the top of my head, a water to freon exchanger is more efficient simply because you don't need the fan for a freon to air exchanger. All depends on the water and air temps though. If you have a pool, that's a great heat sink for home central air cooling. All you need is a little donkey pump for the water to cool the condenser for the home A/C. Lot quiter too. Have no idea what the $$$ savings would be cooling the home A/C condenser with 80+ degree pool water but I'd bet it is significant. It all depends on the region of the country and climate. All I can say is up here in Northeast PA I'd be using the pool water if I had central air for the house. If I didn't have a pool that could use heat I'd be using stream water which is cooler yet. Just my thoughts.
Al