Gotta go with Pepak and Amalthia. Single-file (X)HTML is future-proof, and a great archive format. ePub is still evolving.
Add excellent metadata to your <head>, think about good tag-structure, and you are good to go. You could even use the Dublin Core stuff for meta if you want to nod to ePub, and you'd find conversion a snap.
XHTML is simple to edit and update. ePub is not, split as it is between lots of files, including several metadata files. (ePub doesn't demand splitting files into chunks, but because of limitations of hardware readers, it is becoming the standard.)
'course
I just spent a couple of months working out an XHTML spec, so I'm probably nutty over it.
m a r