I'm brand-new to readers so fwiw:
I went with the 300 over the 600 because 1) I don't know why I'd need > 50 - 75 books in the reader, 2) paid $170 for it, 3) aren't 100% positive what I wanted in a reader yet so felt no need to get all extravagant first time out and 4) did not like the idea of touching the reading surface or lesser clarity due to the 'feature'.
so I went downscale and after using it the past couple weeks really love the thing (price was probably my #1 factor, being new to readers and all). I got the lighted cover since I do a lot of reading before going to sleep, it's a tidy little package and tbh a 6" reader now would probably seem gargantuan.
personally though one can't really go wrong with any of them imm - having a pile of books in the palm of your hand is a great thing