original poster already mentions that glare is not issue to him. so, please, let's not stress this subject anymore since he knows already.
Let's give him any advantage of Prs-600 over 505:
1. searchable, internal dictionary.
you can touch the word to search.
2. Faster CPU - 400Mhz instead of 200Mhz in prs-505 - so it loads the book much faster (specially in pdf). and turning page is little faster.
3. Zoom-in function in PDF. 505 has only reflow function. 600 has both. very handy and effective.
I think these features are worth additional cost.