WHsmiths only sell the Cybook via their internet site, although you can get it delivered to a store for you to collect.
IMHO the Sony is the better looking, but the Cybook has the edge in book availability, especially as the Waterstones store is hopeless. Interestingly, the WHsmiths store now sells books in both the Mobi & Sony epub format. More recent titles do appear to be in the epub format only so things may be changing.
I got the Cybook and am very happy with it, The font changing abilities are a great feature as it means I can read without my glasses if I have to
