JSWolf makes good price related points. Another thing to look at is availability of titles for tje device. Have a look at Sony's store
to see if the have the sorts of titles you're looking for. For the Cybook, look at www.mobipocket.com
and also fictionwise to see how you like what's available in mobi format. If you like Baen books, they'll work fine on either device since the Sony takes RTF files unconverted. You'd of course want to use Baen's Mobi version on the Cybook, though it takes RTF just fine too, so far as I know.