Here's my opinion. DVD writing quality looks about the same to me, which is expected, given the similarities in hardware. The variations you see are mostly within the normal range you get with different drive/media specimens. There are some questionable scans but nothing that I would worry too much about.
On a cautious note, while it's too early to speculate about how the 1640 will turn out eventually when its firmware matures, if the history of NEC 3520 vs 3500, Plextor 716 vs 712, and Pioneer 109 vs 108 is any indication, revisions of good designs have not always been better.
We'll know for sure in a few weeks. In the meantime, if I were you, I would go ahead and get a 1620. It's a solid, versatile, time-tested drive that can be had for amazingly low prices. It has excellent firmware and plenty of patching tools/possibilities to entertain yourself should you get bored with its standard, factory-default behavior.