I love my 600. Yes, the glare can sometimes be an issue, but as has been said by many, if you tilt the device, it goes away. And, after you get lost in a book, you don't notice anymore.
I think glare and $100 are the main issues at stake. It seems that 600 owners who are also first time eInk reader owners (myself included) seem to think glare isn't that bad. Current eInk reader owners (505 owners in particular, it seems
) gather their pitchforks and start a riot about how bad it is. So, it seems ignorance is bliss on this front.
The $100 buys you some extra features, and that is what you need to keep in mind. Are you just going to read ebooks? The 300 should be fine. Are you going to read graphical PDF files where you'd want to zoom in? Does the extra screen size make a difference? Do you want to annotate what you're reading? Then you'll need to pony up for the 600. Basically, if you're just using the reader for leisure, the 300 should be enough. If you're planning on using it for "professional" purposes (as a student or someone who is reading documents for a job and wants to mark them up), then the 600 is better.
If you are interested in the reader just to read books, and you are willing to spend up to $300, I would seriously wait and consider the nook. I was all fired up, but when I found out it couldn't do some of the things with PDFs that I needed it to, and I realized how useful annotations could be with a touch screen, I went with the 600. But the nook has a perfectly clear screen, easy and fast navigation with the capacitive screen on the bottom, and a dictionary (that the 300 lacks). If I was buying a reader just to read books (and not technical PDFs), I would have gone with the nook and never looked back.
That said, I love my 600 and have no regrets.