One difference between Sony Connect store and Mobipocket is that Sony Connect advertises "thousands" - at one point I saw 15,000 as a number - commercial books. Which I think includes their "Sony Classics" line. Mobipocket advertises 39,000 "premium titles" and 10,000 free books.
Don't get me wrong - I love my Reader and wouldn't give it up. But if I was getting it today, I might wait on the Cybook, just from the standpoint of number of titles available for purchase.