I would also toss out the Entourage eDge or the iRex (DR800SG||Iliad).
The Entourage seems ideal for a student. The DR800SG is good if you don't need annotations too badly, and the Iliad would be good for your PDFs. The DR800SG will probably support annotations before too long, but I wouldn't buy it if you NEED that feature. I probably wouldn't buy the Iliad myself because I already have a collection of microSD cards and don't want to diverge from that format.
The PRS-900 is a good option from what I've heard. My main complaint with it was that I tried the PRS-600 in Best Buy and was very unhappy with the firmware. It was so bad that it would drive me batty. YMMV.