I've been looking at those devices at september last year for my self. After testing the PRS-600 for a week, I've returned it and bought a second hand ILiad v2.
When I had the Sony Touch I've tried it in all sorts of places I might be reading and came to the conclusion that the reflections (of my face clothes, and more) were distracting me from the reading. Every minute movement of the device made me see those reflections. The best way to counter them I found was reading with a black shirt on -.-
So I returned it and decided that touch on top of the e-Ink was a no go for me. But having stylus input would still be nice.
Then I went to the Dutch Ebay equivalent and got me a ILiad, slightly used, but for the same price as the PRS-600. I've been happy ever since and after seeing the screen of the ILiad I finally get it why people are so fond of e-ink. (And I've got extra screenspace
)
Since I tend to read alot a good screen was most important for me. The perfect device for me is still not available but for now I can live with the ILiad's flaws (those reflections were far more anoying).
Oh, yes. You wanted to know which one to choose. If I were you I'd go with a device with Wacom, so you wont be disappointed. So any Irex or the Onyx Boox.