Ya, know......I hadn't thought about the relationship of the joypad to the cyBook! That's got to be part of it.
As for comparisons, my impressions to date include:
PRS-505 - heavier, more robust construction, *never* misses a page turn(!), much longer battery life, page turns marginally faster
cyBook - lighter (and so far a bit easier to hold), better font size support built-in, dictionary support (I'm using the Chambers dictionary)
I'm still working on which one I prefer...that'll have to come after more extensive testing.
-Jeff