I think Harry makes a good point - you really have to decide what's important to you before you decide which reader to buy.
Cybook has one button, Sony has two (sets). But Sony is a little heavier. So would your hand tire more from using only one button, or from holding a somewhat heavier reader?
Cybook has the dictionary lookup (so does the eBookwise, by the way, but it's definitely heavier, and it's not eInk).
Neither have great navigation choices for the list of books, but they are different. How many books do you think you are likely to put on a reader? I put 600 books on both my Sony (505) and Cybook. Getting from the front of the list to the end on the Cybook is absolutely horrid. The Sony 500 was worse, but with the secondary TOC level in the 505, it's much easier to find a book fairly quickly. However, if you don't put lots of books on the reader, I wouldn't think this would factor into your decision.
Cybook has the built-in ability to switch out fonts and choose more fonts sizes. (I'm told you can with Sony, but it involves hacking the font file and if not done correctly can "brick" the reader!!
) This is actually a big reason why I bought mine (plus, I was upgrading from the 500 at the time, and the screen resolution is much better on the Cybook - and 505). But it turned out to be more of a gimmick for me. I played with it at first ... but then I settled on a font face and size, and seldom changed them again. I still wish I could get Calibre to imbed a san-serif font to my LRFs that will work (only works with serif for me, so far), but it's not enough to make me pick up the Cybook again.
Cybook's DRM'd format is Mobipocket. Sony is its own (BBeB) and Adobe ADE. I'm still watching for an eInk reader that will natively read DRM'd eReader. (And if you're willing to hack the books you own, the DRM is only an inconvenience factor...)
So, clear as mud?
