You best choice is almost certainly the new Sony PRS-600, which is already listed for $299 in the US, see
New Sony model to be called PRS-300 and PRS-600 (post #244 and the subsequent discussion). The PRS-700 was already the best PDF reader in a 6" device, but its screen was a bit fuzzy and its PDF zoom mode did not quite work. Both of these issues have probably been fixed in the PRS-600, which is clearly the follow-on to the PRS-700.