Welcome to MobileRead, loci!
For JPG manga comics I don't know how much it would matter. The displays are the same size, though the Cybook is supposed to use Viziplex, and the Sony is still using the previous version of e-ink. You might prefer the slightly lighter weight of the Cybook. But you might also prefer the metal casing of the Sony.
The other differences mostly don't apply to JPG/Manga, I wouldn't think. The Cybook's swappable battery and expected larger number of supported formats don't make much difference.
The question might come down to what you can get hold of most easily, but I'd think each would be about the same as the other for that purpose.
I would suggest that you handle one if it all possible before you jump (and take a sample file along with you on an SD card) -- that way you'll have the best possible idea of what to expect.
