I've owned a Kindle 2 & a Sony 505, and eventually found that I liked the Sony better. A lot of it had to do with size. So tonight I was in a Borders and took my first look at the 300 & 600 models. (No question but that the 600 is much easier to read than the 700.)
What surprised me was how much I liked the 300. And I think it has to do with size again. My impression is that the 300 also has greater clarity than the 505.
I plan to take my 505 down to the store tomorrow evening and do a side by side.
There's a fair chance I'll buy the 300. The great advantage of ebooks - for me - lies primarily in portability, and the large part of that advantage lies - again for me - in getting the device down to the smallest size possible consistent with reading comfort. Reading on my iPhone is doable but a bit too small for comfort.