For me the benefit of an e-reader is the e-ink display, also low energy usage and portability. I also enjoy having a dedicated eink reader that does not do web browsing and all the wonderful features of Android. Ideally the feel of using an ereader should mimic that of reading a book.

A benefit to using an android tablet would be the ability to torrent books and read the web and do file management with .zips and stuff all on one device. And if you plan to read color textbooks PDFs you might prefer to use an android with a larger display and better multitouch zoom functionality.