I have a BenQ 1620 and a NEC 3500, the BenQ works fairly well with almost all media, plus and minus, and allways produces very readable results. The NEC however does not work well with as much media, and does not seem as stable as the BenQ 1620, but it does a better job with minus media then the BenQ. I have not really tried either with CD media, as I use a Plextor Premium allways to write CDs anyway.
Personally I cant see much point in having two identicle drives, unless you into mass replication and are burning more than one disc at one, or are using two machines. So I would stick to having both the BenQ and the NEC. As far as I can tell I got a "good" BenQ 1620.
Edit: I just tried my NEC with some TY CD-R media, that work at 52x with my Plex Premium, and I think officially are 48x, the BenQ allowed them to burn at 40x, but the NEC only at 32x (with Nero 6) so maybe NEC is having some CD-R media compatibility problems.