I remember reading on some sort of specs page for the PRS 900 that you will be able to zoom in to cut out margins... please don't quote me on that, as I'm not entirely sure I read correctly; however, if that is true it would be great for technical pdfs. I read a fair amount of journal articles and non-fiction (school) pdf on my PRS 505 and the reflow is great, I have no problems reading on a 6 inch screen. I can even read scanned pdfs and reflow the text if it is scanned with character recognition. I don't know if the nook has pdf reflow, you should try to check that out somehow before deciding.
I guess your choice boils down to the type of pdf's you read; mine are of a technical nature, but not TOO graph or picture heavy, and I am very happy with my 6 inch Sony. I also use programs such as pdftolrf if I come across a scanned pdf that is too small to read on my reader. The PRS-600 has a zoom feature that might come in handy for that sort of thing, and I imagine handles pdf's better than my 505. You might like the added note taking ability with a stylus. However, I'm really considering getting myself a reader with that ability and will probably be checking out the prs 900.
Best of luck in choosing, I'm sure that you will be happy with your reader, whatever you decide.
Last edited by Latinandgreek; 12-22-2009 at 05:41 PM.