I don't think either device will be all that great for reading PDF files, frankly. I have a 505 and I love it, but use it almost exclusively for reading LRF and ePub files.
But it really depends on how your PDFs are formatted. I'm assuming that they are formatted as a4 or letter size. If the PDFs are formatted to fit on the smaller screen, they'll look okay, but I'd lean toward to 505, just because the screen is bigger.
You might want to wait to see what the 600 has to offer if you'd like to make annotations. From everything I've seen, it looks like it'll have a touch screen which will probably allow annotations.
Good luck with your choice!
dordale