One reason that format matters to me because I've paid attention to the results of conversions back and forth, especially the DRM-stripped kind. I've tended to have better results with epub to mobi than the other way around, and I wouldn't be surprised if the same applied to epub 2.0 and Format 8. So far, Apple's ebook format is the most recalcitrant of the lot, but I still have about four Kindle books that will not make the leap -- particularly Charles Bernstein's
My Way and
The Madman and the Nun and the Crazy Locomotive, by Stanislaw Witkiewicz.
(BTW: Bernstein is a language poet and deliberately employs typos and homonyms to humorous and/or annoying effect. You don't want automatic spell-check anywhere near his books!)
Truth told, I also like the aesthetic results of epub to mobi more than mobi to epub. I wish I didn't, since Amazon's the most convenient store of them all.