I guess the books are purchased from Sony are DRMed.
I'm still hesitating to use Dropbox, but I am more and more seeing the benefits of it.
I had wondered if a Sony app with access to the Sony reader store could peacefully co-exist with a Kobo app with access to the Kobo store, but I just installed the Kindle app and it seems to be doing OK. (Interesting to notice that a book I am interested in is $5 more at the Kindle store than it is at the Kobo store.)
Sony books are usually a little more expensive than Kobo books, but, as I've said, sometimes Sony has titles that Kobo doesn't carry. It's much easier to navigate through Sony's free selection as well.