Choose AC version if you have a 24VAC pump start relay (the reason is that the DC version is known to not work well with all pump start relays).
Choose DC version if you want to power the controller off-grid (like using a solar panel), or if you can’t find a 24VAC power supply locally (the one we sell is only for US/Canada/Mexico, if you are outside of North America, it’s often difficult to find 24VAC power supply; in contrast, DC power supplies are much easier and cheaper to find).
If you don’t have a pump start relay, the DC version is a cheaper option and works well with almost all 24VAC valves.