I like Epub on both. One thing I've noticed is the PP does full justification on many if not all of the books and I don't care for the way it does it, adding extra spaces etc make it hard to read. I think.
Full justification in a book is different cause it doesn't add full spaces.
I try to avoid PDF on both of them but do have one short story book in pdf that I've been reading on the 505 -- it's passable, but I don't like the way it does page breaks.
The PP will of course handle more formats -- like MS Word -- which I've tried briefly with results similar to PDFs.
I like that the 505 always has the page number and battery level displayed. I think the UI and navigation on the 505 is head and shoulders above the PP -- you can search and find my comments on that elsewhere.
Don't get me wrong - I like the PP, it's just that I like the 505 better and think it is overall a better reader for my particular needs.