I'd wait for the K3 and keep reading the reviews as some of the K3 buttons seem to be loose and falling out and some folks are looking at long wait times for replacements. You get what you pay for as the saying goes. I have had both K2 and prs600 and have lots of books in both formats, although I removed the K2 DRM and converted with Calibre so I could read the books on my Sony. Both are good at saving notes and highlights, but Kindle is better at keeping them online for you, where Sony requires a sync with the Connect software which is buggy as hell, and their books tend to be more expensive than KOBO or AMAZON. Download the Kindle for desktop and play around with the notes on a sample or free book and then check out your notes on the following site to see your notes and the summary of highlights that other folks have made on the same book:
http://kindle.amazon.com/home/welcome
It's all a matter of preference, I guess.