As someone who came from the Sony camp (a PRS-500) to the Kindle 2, I'd recommend going with the Kindle. The wireless makes all the difference - from the auto-downloading of subscriptions to periodicals (NY Times without all the Calibre hassle, albeit at a higher price) to being able to browse books, and even more importantly to sample them for free, without having to use a computer makes the whole thing a seamless experience.
I still read books from my own collection that I load via USB or wirelessly to the Kindle, so I still have the same experience on that front. It's just when I want to read something that's brand-new (like my most recent reading -
The Strain) I can sample it, order it and get it for $10.
I also think that the K2 is more comfortable in my hands and that the page-turn buttons are better placed for ambidextrous use.