If you don't need the flexibility of Android and don't read PDF files, go with the Libra 2 (it has the best screen and page turn buttons and Kobo seems nice to use).
Whatever you do, don't go for the Nova Air. It has a pre-applied non-removable screen protector that seems to really decrease the contrast. It has lower contrast than the Nova 2/3, which is already lower than Kindle PW3. The Poke (and probably Leaf) have screens as good as the Kindle PW4 from the images I've seen - but all of those are worse than the Libra 2.
(See Nova Air screen comparison here
https://www.youtube.com/watch?v=LqKDdImVaFE)