I have a trusty "old" 1620 that has done its job very, very well. I just got a 1640. I can only keep one drive. From my tests on TY T02, MCC004 & CMC E01, I would say the currently 1620 has a slight edge. More importantly, the 1620 safely overspeeds quality media such as TY T02 stock, whereas with the 1640 I would have to use MCSE, which I'd rather avoid if at all possible (not currently possible with 1640).
I understand that there is a high likelihood of the 1640 being improved into a better supported drive via post-BSJB firmware updates. I have had the 1620 for a while and I consider BenQ to have rather high standards as far as quality of workmanship and firmware support is concerned.
So, which would you keep?
Here are a few 1640 scans: first two are of MCC004, the first @ 16x and the second @ 12x -- I consider the 16x to be crap quality, and the 12x acceptable (1620 burns it similarly, so the media's to blame). The last scan is of Fuji (1133 hub) TY T02 @ 8x - good PIF, high PIE considering the low burn speed.