I don't have either of those readers, although I love Calibre for managing my collections and conversions, etc.
Do you really need to keep so many books at once on your reader? It seems like if you can access Calibre easily, then you wouldn't need to keep all your books on the device - unless you keep a lot of reference books, manuals, etc on it.
I use a 7" Android tablet and my old 2nd Gen ipod touch, and both can access my Calibre collection via local servers (actually, one server is available via the internet if my computer is on).
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