Etextbooks and readers for students just aren't here, yet, in my opinion.
I have a Sony 500 that I love except that I can't buy any textbooks for it and if I scan the pbooks myself (an incredible chore) I end up with PDFs that I convert using pdflrf (available from this forum) which works but each file is many Mb and they are not at all ideally viewed on my Sony. It is difficult to flip back and forth between sections of any ebook on it and there's no searching or highlighting available on it. So if this is your primary goal, I would look at an Iliad (I don't have one) or else a laptop or UMPC but only if you are willing to scan the pbooks yourself. Ughh. You can "rent" some textbooks from CourseSmart in ebook format but you can only read these online and the cost is half the list price of the pbook for only 180 days.
The Kindle might change this situation. For example, I see that Amazon offers well over ten thousand titles in their Science category including some that lead me to believe that they may soon offer real textbooks in Kindle format. Sony and Mobipocket, at least today, are still way behind in this category and they offer no content that resembles anything technical at all. If Amazon continues at the forefront of content offerings, then it's just possible you will be able to buy etextbooks from them but if so you will only be able to read them on a Kindle. I'd recommend that you see if the textbooks you're likely to need are listed there. Also, the search and highlighting capabilities of the Kindle would make studying more practical on it than for on a Sony.
So for me, my Sony is great for novels and such. I have some technical content on it but I can't do serious studying with it. I do some studying on my laptop but mostly on material I have had to scan myself. That means most of my studying is still done using dead tree books.