The more I poke around teh intertubes, the more confusing the choices become. The biggest problem seems to be what features the various reading programs support. In our rush to separate semantic content from layout, readers have ignored or thrown out all the previous effort that has gone into getting great layouts (even on multiple devices). With the switch to XML, our readers have automagically forgotten all the programming that went into reflowing text, pagination, device independent output, hyphenation, and typography available in the TeX libraries alone.
Many publishers have put their name behind ePub, very few have published anything to ePub. In the meantime, they are more likely to use a hand-rolled XML format internally (even feedbooks admits to his
).
TEI looks interesting in that it allows an author to use inline TeX to express math formulas. The rest of the formats tend towards MathML. And DITA hasn't settled on a math domain yet.
DITA has the advantage of allowing very non-linear documents. As Jon Noring points out, just try to express a website or Wikipedia in ePub. In the Palm days, the database program TomeRaider did a very good job of handling both dictionary content and linear narrative. DocBook, TEI, and ePub all tend towards a very linear approach.