It is possible to replace the battery for Air, more or less as a matter of course but it is always reassuring to know for sure before you purchase.
Onyx told me cost is depending on the warranty status and where device was purchased. For example, they have a repair center in Poland. Price seems reasonable
https://www.reddit.com/r/Onyx_Boox/c...nyx_boox_note/
Your retailer can be your first help as well.
Given that, battery is secondary to me. I think Air battery is 20% weaker in terms of time than N3, still ok. Air has other pros. Weight is balanced by handle. Screen should be better. Storage and storage speed is secondary to me. Performance should be the same with normal text. Gyroscope Air. Taking notes not a premium to me and difference not that big. I would probably prefer Air to N3 even if there was no price difference.