I chose the CyBook because A: I own one
and B: mobi-support is until now superior and that's what counts for me in an e-reader. I'm also not sure if the BeBook has dictionary support and that's a big plus too for the CyBook.
On the other hand I realize that the BeBook has far superior power-management and that they are closing the gap in mobi-support rather quick.
I bought my CyBook when the BeBook had no mobi-support at all. At the moment I guess I just would buy the cheapest .......