I've looked at the Sony PRS300 during my research and what I didn't like about it is it's limited storage with no expansion slot. I know this isn't an issue considering that ebooks only ranges up to approx 3MB but still I would like the option to store books in an SD card for convenience.
I agree with Russel and suggest you consider the Jetbooks, too.
I've had my jetbook-Lite for 5 months now and it was the best device I've bought, like ever. Budget was an object for me also at the time and after hours of research, learned the jetbook-Lite has everything I need at the right price ($119, free shipping at Newegg).
What I like about the Lite other than the price, is that it uses 4AA batteries which enables me to replace them easily with an extra set when power is drained and I don't have to plug and wait for it to recharge. It can also read numerous ebook formats. It also comes with an SD card slot, so transferring files is convenient, too coz you can do it wirelessly with a card reader.
However, you may also have to invest in AA battery sets if you don't have them in stock and the screen of the jetbook-Lite has a grayish tint to it as oppose to the more white-ish screen on readers that uses e-ink. The pro in TFT screen on the Jetbook however is there is no flash in between page turns. It is instantaneous.
Just my 2 cents.