You truly can't go wrong with either drive. You need to assess your needs. I reviewed them both (links in signature), but I will give you the bottom line:
The LiteOn 1693S is a great all around drive. It could read faster, and it had a problem writing a DVD-RW disc. Other than that, it does very well on a wide variety of tasks. If this is your only drive, it is a very strong choice.
The Benq 1640 (I tested the Plextor 740A which is identical except for firmware) is a really fast reader. It has slightly better writes on DVD+R and DVD-R media. It also does well on both + and - rewritable media. It had trouble on the DL DVD+R disc, but others haven't reported this problem.
Both drives are quiet, ship with Nero, and have shorter chassis.
Good luck with your choice!