I recommend Android. The hardware is significantly cheaper and Moodle does not require any of the possible hardware advantages of of iOS devices. I speak as someone with a 2018 iPad, but that is because is really good at audio/music so no advantage for Moodle. I recently purchased a pre-loved Amazon Kindle specifically for using/testing Moodle mobile and even shiny new they are very cheap.
The other subtle advantage is that there are vastly more Android devices in the world so you can be confident that the Moodle App and any plugins will have been used more/tested more on Android.
I used to work somewhere that had a rack of iPads and they seemed to require a great deal of management, particularly as they are at heart single user devices.