IMHO it's all about money and "style". The oil cooling system is significantly less expensive to produce. Of course the old adage applies, you get what you pay for. There is no doubt in my mind that the water cooling system is more effective at controlling head temps. But as designed by Harley it requires lower fairings for the radiators, more plumbing, fans, a thermostat, and pump. The oil cooled system requires a big oil cooler and different oil pump, and they don't even give you a fan for the oil cooler. They will sell one to you however.
If Harley wasn't so afraid to put a proper radiator on the front of the frame downtubes, like some of the competition, you wouldn't need lowers to hide two smaller radiators. As usual, their decision making process gave a much higher priority to style and appearance than to actual function.
If you go with a bike that comes with oil cooling, I would definitely suggest you add the fan for the oil cooler. The cooler doesn't do much for you if it doesn't have plenty of air moving through it.
Jerry