I tried that. I didn't notice any difference.
I ended with the dictionary on the SD card because I can live with books in the main memory and 'loosing' the SD card and dictionary. The other way around will leave me with a dictionary and no books
This saved the day when I was traveling in Asia and the SD card from my camera died. I simply cleared the one from the Cybook and placed it in the camera. The Cybook still had all the books in the main memory