Day is usually preferred as you are generating additional chlorine when chlorine consumption is at its peak. SWG pools use a lower ppm of chlorine and a higher CYA. If you ever get behind on chlorine production, and an algae problem develops, it takes a bunch of chlorine to get to shock level and clear the algae. As mentioned, you can also split runtime between dark and sunlight hours. My folks run their SWG pool from the wee hours until just before noon, which is when the rates go up. They have their SWG set so that when they measure chlorine at sunset, they never drop below the minimum recommended level.
We tend to run during the day, especially when using the HeatPump to warm the pool. When we get to the point where we need to run the HeatPump in chiller mode, we will start the pump in the wee hours to take advantage of cooler temps for chilling.