Be aware that both readers are not optimal for reading pdfs - especially if they have intricate layouts (e.g. several columns) or copyright protection. In these cases you are better off with the 600 because the screen is a little larger and it is easier to zoom and move around.
If you have one flat file without complicated layout and copyright protection then the 300 is good enough. In this case there is always the option of converting the pdf into a more adequate format (e.g. with Calibre).
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