Shopping around for prices on books pretty much implies Mobipocket, as it's the most widely available format. This, in turn, essentially rules out the Sony Reader and points heavily towards the Cybook.
As for e-books, if you like Science Fiction and Fantasy,
Baen has about a hundred available as a free download and many more to buy. There's also the Baen CDs available as a download from
Fifth Imperium, which raises the number of freely available Baen e-books to roughly 200.