The screen on the 505 is somewhat better than the 600, I agree, but the 600 has a
very rich feature set - annotation, search, dictionary, etc - which for me personally make that a price worth paying.
You can see from my profile that I own a fair number of different e-Book readers

, but if I had to choose only one from that list to keep, it would be the PRS-600, no question about it.