If you want to read books, then probably any 6" EInk device will do. Practically everyone except Amazon is switching to Adobe ePub, which means that the reading experience is going to be very similar on all of them. It is still too soon to tell if there is actually any important differences between ePub on (say) a PRS-600 and a EZ Reader (Hanlin V3).
The new 5" EInk devices are interesting, because this might be a more convenient form factor. However, the software is identical to the 6" devices.
For letter/A4 PDFs, 8" EInk screens are much better than 6" screens and ~10" screens are better still. This is because pan and zoom does not work well with the slow refresh rate of EInk. The Kindle DX has crippled PDF support, and I can't recommend it (although it does demonstrate than this screen size is going to be very good for PDFs). The DR1000S is very good but very expensive. The new iRex 8" device,
Mainstream iRex 8.1-inch reader for 400$ (rumor), looks promising since iRex has good PDF support today. It is likely than several companies are working on 8" and 9.7" devices, because these are standard PVI panels, but are waiting for "plastic" PVI screens vs the glass screen probably used by the Kindle DX. These screens should be available this fall, but they have been delayed before so who knows when they will actually come out. I would still recommend waiting for them, becuase they are all going to support PDF better than the Kindle DX.
If you must read letter/A4 PDFs on a 6" device, then it is likely that the PRS-600 is your best option. All the Adobe ePub devices also use ADE for PDFs (including Adobe PDF ebooks), but the PRS-600 (follow-on to the PRS-700) is the only one currently with a touch screen and a PDF zoom mode as well as a reflow mode. However, the zoom was not very practical on the PRS-700, and we don't yet know if has been improved on the PRS-600.