It is likely that the new PRS-600 will have the best PDF capability in a 6" device, with (like the current PRS-700) both reflow and zoom options. However, "best" isn't "good" or even "good enough". You can get an idea of the PRS-700's capabilities from
Sony's Windows ebook Reader. It is likely that this software will also be upgraded when the PRS-600 is released sometime this month.
If you can control the font size, printing web pages to PDF might work well for new PDFs. You would want a very large font size though (24 pt for a letter/A4 page), and this probably isn't true of your existing PDFs.