Think about how you shoot most. Close, quick work favors a dot that's easy to see; slower shots at distance favor a smaller dot that covers less of the target.
Mount height matters when folding irons stay on the rifle. Check whether you want the irons centered in the window or sitting low, and pick a mount that gives you that view.
Keep a spare battery in the stock or bag, and check brightness settings before each trip so the dot isn't washed out in daylight.
Folding backup irons are often run under an optic as a co-witness backup. If you're choosing an optic to pair them with, see the Ozark Armament Rhino red dot sight.
Rather than tracking down separate product links, you can open their Amazon store page, which gathers Ozark Armament's AR-15 iron sights in a single view. Flip-up backups and fixed sights both show up there.