The 300 only has three font sizes, the battery is not easily replaced, does not have SD slot (all of which the KOBO does have), but it is a better screen for clarity than KOBO reader. However, if KOBO keeps updating the software, the I think it would be a better reader and I like the larger screen. Neither have touch screen, nor do they have notation/bookmarking text which you may miss after using the 600.
I am waiting for SONY to bring out their new line of readers and will most likely go for the 650. Five inch screens are just too small for my old eyes, and I can read books on my Droid phone for short periods of time if I want more portability.
It really boils down to what works best for you and what you are able to pay. A current step up from the 600 would be the boox/neo, which I do not own, but looks like a really nice reader but price is about the same as SONY 900. Here is link to boox video:
http://www.youtube.com/watch?v=FJUNj...eature=related
If SONY does not come out with a better reader than the 600, I am going with the boox.
Hope this helps you.