The PRS-300 doesn't have an expansion slot, so that one should be out of the running...
I can only talk about the Mini. If you want large lettering, I'd go for a larger screen. Simply because the screen is bigger and thus can hold more letters on it. If you want normal sized letters or even smaller, I'd go for the Mini. It's newer and should be faster. And also because it supports folders (which I understand the Sony doesn't have, that one does have "collections", don't ask me how that works...)
I was used to reading on my 3.4" PDA and the screensize of my Mini surprised me, in a positive way.