The Ring Alarm base doesn't have high data needs, so speed really isn't a major issue. Reliability is what is key.
If your base is close to the router and you've got lots of 2.4GHz devices and networks, 5GHz would provide an ideal connection since it's less prone to interference. But if you're locating it far away from your router, you may find you need to use 2.4GHz.
It really just depends on your layout, the proximity of the devices, and which connection works better in your exact circumstances.
I have mine on 5GHz, but I've installed them on both network types without issue (and wired networks as well).