i'm currently a owner of both drives in question (resident BenQ slut at your service:))
i only do dvd backups though so I can't really vouch for cd or game copies, but i can say that they are both so good, it's tough to choose which is better. I just purchased my 1655 today and i'm getting burns just as good as my 1640.
if you can't find any info about cds and games to sway you to one side or the other, i'd be inclined to recommend the 1655. The main reason I bought mine was because I wasn't sure how well the 1640 would be supported with firmware updates and whatnot now that the 1650 and 1655 are available.
Ben Q has shown no signs of giving up on the 1640 users (or even prior drives) but for the sale price at staples this week ($41 before tax for me) it can't hurt to be safe