If you have a BestBuy around, you can do some compare/contrast between the Kindle 3 and the prs-350. Honestly, it was a bit of a coin toss for me when I was trying to decide: both have gorgeous screens and the size difference really wasn't that big of a deal. Being able to download books on the fly was the deciding factor for me--I tutor literature and often have students asking if I've read this or that random book.
Looking at your criteria and how well/poorly the Kindle matches:
Responsive and Fast Page Turn: Yes! Yes! Yes! (how's that for technical?)
Screen contrast: It's the same as the Sony. Both are really, really nice.
GUI: The Kindle doesn't display book covers and you can't organize books into sub-collection folders. However, I found the menus to be easier to navigate than on the Sony.
Comfortable for long reading sessions: I have no problems holding the Kindle, but my husband found it awkward. If you have large hands, it might be a problem; however, using a case (I have the lighted case sold by Amazon... love it!) can give you a bit more to hold on to.
ePub support: No. But you already knew that
I presume you know how to "liberate" your DRMed ePubs?
Wifi/3G: I have several news feeds scheduled on Calibre; it's nice to have them download to my reader when I'm out and about. Being able to download books anywhere and everywhere is also nice. Oh, and the other day, when I somehow managed to get completely lost in downtown L.A., having 3G and a web browser was a very good thing.
Keyboard: I'm pretty meh about it... buttons are a bit too small to type comfortably. However, it's doable and they really don't get in the way (you have to press fairly firmly on them to type, so you're unlikely to accidentally type something).
Touch screen: If I could afford the prs-950, I would be all over it for the Touch Screen and the ability to actually write on the screen--it would be fantastic for marking up student papers. However, I've become so used to the speed of my iPod's touch screen that the comparative sluggishness of the e-ink refresh rate just felt a bit disappointing. I just kept saying things like, "ooh, this will be fantastic when the next generation model comes out!"