I'm struggling with the same problem (though it's not physics ^^).
Size is one thing. If that's your only concern, then any reader with a 9" screen should do the trick. The Kindle's support of pdf used to be dreadful. I'm sure it's better now, but having to install a new app for something that should be a basic functionality...
If you want to annotate your pdfs then the M92 seems better (though no OCR...). I might be biased on this one though because I don't like Amazon's e-book format policy. Still, having a wacom stylus should beat a capacitative screen for note taking.
What really makes me hesitate is the following:
- color: my textbooks have lots of colored graphs which would be terrible on b&w e-ink screen
- multiple files opened at once (the M92's new firmware seems to allow this)
- jumping between pages (but it's the same on a computer)