Thanks for the feedback so far.
We're currently leaning towards a combination of Song, Kai, and probably a pen (rather than brush) version of Xing.
Song is kind of boring, but it's the most popular printed Chinese style for a reason - it's extremely readable and regular and fills up spaces nicely.
Kai is beautiful, but awkward for large amounts of reading - as a screen text font it tends to look small and murky. (seriously, try reading a book in it and see how long you last) So I like it for showing headwords at the top of entries, but I think by default we'd probably want to confine it to only that and not also use it for body text.
Xing would primarily be used as a cursive teaching tool - I doubt very many people would want it as their regular reading font, but I can certainly see it being helpful in flashcards and Character Info. And this is why we'd go with a pen rather than a brush version - it's harder to see the strokes in the latter even if it is prettier to look at.
FWIW, we're also looking at adding a really great text serif font for English and Pinyin; we don't really like any of the built-in options on iOS, and on Retina / XHDPI displays we think a move to serifs makes sense both for readability and aesthetic reasons. (though we'd certainly retain the current Arial as an option)