I own both a 505 and a 600. The 600 is my third dedicated device, which means I was used to the better screen quality of the previous ones. Still, I use my 600 almost exclusively now. I love its extra features, the natural way to turn pages, the ability to annotate (which I didn't think I would need), and definitely the dictionary. I almost never use a dictionary with paper books any more, but the ease of using it on my 600 means I can look up any word I am in doubt about without any hassle at all, with the definition even appearing on the same page while I'm reading.
The search option is also something I missed on my previous devices. Maybe my memory is not what it should be, but I sometimes come across characters in a book that I'm sure I've met before, but can't remember when or where. The search option comes very handy in such cases.
I once tried to read a manga on my 600. Mangas being images instead of text, I couldn't use many of the 600 features, so I thought I would be better off reading it on my 505. Indeed, visually it was much much better, and I always read comics and mangas on my 505 now, BUT! it was then that I noticed the huge difference of speed. On the 505 (and most readers really), you press the next page button and wait for the processor to realize what is to be done. You actually have the time to read a small paragraph before the page turns. The fist few times I read on my 600 I found I had to go back to read the last sentences, because the page had changed much too quickly!
All in all, I would definitely recommend the 600, especially if it is your first device, unless you are absolutely sure you don't care about any of the extra features (and bear in mind that even if you think so, you will grow to love some of them)