The advice about pdfs is good. The advice about holding each for a while is very good.
I've messed around with the 300, the 505, the 700, the 600 & the Kindle 2. They all have their strengths & weaknesses (except the 700 - all weakness IMHO.)
But the 300 is just perfect for me. Fits my hand, & the screen ratio seems proper. I find myself using it much more than I ever did the 505 I wound up buying before. The size differential makes a big difference, somehow - I take the 300 with me where before, I would not have taken the 505.
I intend to use my 505 at home, when Instapaper upgrades so that I can move web articles to it:
http://blog.instapaper.com/
Do not let a lousy ten bucks decide this for you. Paying less for a reader which you use less because you like it less is not the way to go.