that's exactly it.. with the kindle app, the books may be DRM'ed, but at least kindle is available on a number of platforms,
and amazon is likely to keep developing kindle applications to run on future systems. apple's books are locked into their bookstore on their devices.
the best choice, if available, is something like
Kobo Books because not only can you download .epub's but most books come in a few other formats like .mobi, .pdf, and even read online.