OpenSprinkler DC is better for International users mainly because it’s often hard to find 24VAC transformer. If you already have a 24VAC transformer, then either AC or DC would work fine. But I often get questions from International users about where to find 24VAC transformer. The DC version is designed particularly to address this issue.
OpenSprinkler DC comes with a 9V 1A universal power adapter, and you can easily substitute it with a different power adapter, from 6VDC to 12VDC — it’s fairly tolerant to a wide range of input voltages, and DC adapters are much easier to find than AC adapters. Also, DC adapters are usually regulated and universal (which means it accepts any powerline voltage, from 100V to 240V). On the other hand, AC adapters are very specific to the country’s powerline voltage — you can’t plug a US/Canada 24VAC adapter in other countries, as that will cause the output voltage to double.