Hardware-wise those two devices are very similar.
The difference is the layout of controls and the lid.
Opus does not have a lid and has 4 way D-pad at the bottom. PocketBook has D-pad at the side, between two pageturning buttons.
Software-wise they are VERY different.
PocketBook has dictionaries, on-screen keyboard, support for third party programs, lots and LOTS of configuration options. My PocketBook has crashed (frozen) once, and I was not able to reproduce the crash. All this was during my furious testing and trying to find limits of the device. During the few weeks that I own PocketBook there have been several firmware updates with meaningful improvements. Manufacturer listens to customers and "Guys from Kiev" have even incorporated improved version of FBReader compiled by one of our members. PocketBook has supplied us with Software Developement Kit, complete with device emulator, so you can test your binaries without loading them to the device.
Cybook Opus, on the other hand, is ... different ...
Search forums for "Cybook firmware update" or "Cybook crash" or
see this thread