The PRS-300 and PRS-505 are so similar that I'm sure Sony is discontinuing the PRS-505. There is no point in having three models between $200 and $300.
I agree with
igorsk that any 3rd Sony Reader will be a "900". I guess (hope) that this is just like the 600 but with a 9.7" screen. This is the large-screen device that Amazon should have produced, except that they are clueless about PDFs. So far as I know there is no hard evidence that a PRS-900 is even on the drawing board, but it would make sense for Sony to compete for the large screen market.