I'm wondering the same as you, and I'm leaning towards the PRS 900 at the moment, especially with the price dropping to $300.00.
The reason I'm leaning towards the PRS 900 is the landscape mode with the PDFs, to me, it looks like it handles PDF formatting a lot better than the PRS 600 due to the bigger screen size.
This video pretty much has me leaning towards PRS 900:
http://www.youtube.com/watch?v=MF3p55C1Nt4
I'm still reading/researching though before I pull the trigger. All my books will be in PDF, as I don't really care about shopping for books, so Nook & Kindle having the bigger selection doesn't really much matter to me. If I do get the PRS-900, I probably will keep the 3G permanently off. Like I said, all my stuff will be PDFs (magazines, books, manuals, comics, etc), so a reader that can deal with PDFs is a must for me.
Let me know what you decide though and why, as I'm still deciding PRS 900 or PRS 600 (only because of the price is why I'm still considering it).