In my opinion it's a tradeoff you have to make for yourself. Yes, from what I have seen the 505 is clearer, or at least has more contrast than the 600. Main reason for this is, of course, the touchscreen on the 600.
Is that touchscreen worth it? For me, definitely. I use it extensively to write notes and annotations during lectures and highlight parts of books when studying - not even so much to read up on later, but as a tool to help me remember (I learn something easier when I'm writing it down). The PRS-600 is the ideal scratchpad in that regard (which saves a lot of poor trees, believe you me).
While the contrast is somewhat less than with other books, it is still very well readable (though this being the first eReader I have seen, I might be somewhat biased here) - I have yet to encounter a situation where I had to strain my eyes to read the screen, which is really all that matters for me. In that regard, it is clear 'enough' for me. Whether it will be for you is a question of personal taste and preference I guess.
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