Epub or Adobe Digital Editions: My biased opinion (and I am unanimous in that)
Epub:
1) I find there's too much inconsistency with this format; in fact, some of the files simply aren't put together correctly so reading them on the Sony Reader isn't even possible until/and unless an editor (or compiler decides to fix them. (See
Proust's Remembrance of Things Past from Feedbooks.)
2) I don't like left justified margins. It seems counter-intuitive. I like left and right justified margins.
3) The Small font size, for me, is just a tad too small. However, going to Medium size the font is too bulky looking.
Adobe Digital Editions:
1) They always seem to me to look better on the Reader, perhaps because there seems to be a shared consistency with appearance.
2) I like the font size when it's on Small, which is exactly perfect for my tastes.
3) The files are, of course, larger but with the memory that the Reader has this really isn't a problem for me.
This, then, is
my opinion.
Your opinion may differ. (See: I even underlined "Your" so you won't feel slighted

).
Thanks,
Don