The difference between the cybook and sony is (judgment based on my paper models) is that the sony is slightly shorter and a little bit wider. That does give it little bit more appeal tho... Button placement also looks nicer, you have a round nav button on the left and two nav buttons on the right so however you hold it, you should be able to easily navigate pages (in contrast to the cybook). All of this is based on hearsay however, I haven't actually tested this
Wrt to the A4 reader, if it would be foldable and thus easy to transport, I'd get one in an instant
. But alas, it is not to be (yet)...