Returning to the OP's question, this should really be an easy choice. Neither of these books compare directly.
If the 300 meets your needs why even consider the Kindle? The price advantage is huge. However, for me, the choice at this price point would be the Astak EZReader Pro.
www.theEZreader.com, and I have one on the way for my son.
Now let's consider the Kindle. I own one of these and I am quite satisfied with it. The screen on the PPro is about the same size as the Kindle because of the margins that Amazon puts around books. Therefore, the only real differences are 1) Whispernet and 2) The Amazon store.
I love the Amazon store, the ability to sample chapters of new books, the ability to easily download books. Although Sony shows signs of catching up I'd be surprised if Amazon doesn't manage to build upon its lead. Amazon is about books and I expect them to deliver a good product in this realm.
Whispernet's ability to connect to the net has been quite useful for me. When I was traveling I used it several times to check on locations for nearby places. You should be aware that Amazon has advertised the Kindle as having 'free wikipedia', but the overall free internet is 'experimental' so Amazon could pull this feature other than wikipedia.
The one caveat on Amazon as well as Sony is that I don't trust them over the long run. Amazon was in ebooks before and they pulled their store. Sony was in music/mp3's and they pulled their store. In either case if you buy a DRM book you are SOL if the provider shuts down the store. Any books you don't have in a DRM-free format may suddenly 'disappear' on you at any time.
Now let's widen our circle to add the 600. Its advantage is annotation. If you write in your books then you won't have another choice at the price and size -- For more dollars and with a bigger screen you can find this in the Irex.
Bottom line is:
- Cheapest, pocketable, go 300 (or I prefer the PocketPro)
- Easy to download books/samples. Bonus internet (while it lasts) go Kindle
- Annotations (if you write in your books) go 600