If your library has a great deal of PDF and little in the way of Mobipocket and no ePub, then go with the 505 or 700.
If your library has a lot of Mobipocket and no ePub, go with a Gen3.
If your library has lots of PDF and ePub, go with the 505 or 700.
The 505/700 can display DRMed PDF and ePub. The Gen3 can display DRMed Mobipocket.
If your library does not support ePub, ask them if they plan to because if they do, then the 505 or 700 would be your best choice in the long run.
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