What kind of -R? If you are using only TYG02 then I would go for BenQ 1620 because it's cheaper. If other than that and price is not a problem, then go for Plextor PX-716A since it burns most +R and -R very good. BenQ 1620 is known strong with +R.
Or just like chas0039 said, might be worth waiting for the 1640 because from what I've seen, is better in handling -R compare to 1620.