I compared my 505 with a 600 at the airport asking myself exactly the same question.
The 600 screen isn't as clear. But, if I didn't have them side-by-side, I'd have been happy with the 600.
I am worried about smudgy fingerprints on the 600 - I find they make the 505 harder to read, and I clean them off immediately. That will be harder on the 600, and the rep and I made it pretty grubby, pretty fast.
The dictionary is outstandingly well implemented.
Less convinced about the notes, and a bit horrified about the idea of a stylus.
Finally, all the new features have to be a nasty drain on the battery.
I didn't buy it for three reasons: a) my 505 really is just a fantastic device (and it's only seven months old); b) there is a recession on!; c) it's not available in red, and I don't like the silver strip below the screen.
I probably will upgrade, but not until I've held a 900. In the meantime, I was much more taken with the 300 than I thought I would be - very nice little pocket reader.
I think new buyers can't go wrong with any Sony devices on the market currently. As a 505 owner, I recommend potential upgraders get them side-by-side, your mileage may vary
amjb