3500 = 3500A = 3500AG etc ...the name suffix is generally added by the stores, and do not define any special difference, besides colors and internal references of the store. It's probably the same thing with the BENQ 1620.
The BENQ 1620 is known to be pretty bad with -R discs, afaik. I own a nec3500 and I haven't had any problems either with -R or +R media. I know, because I use it, that the NEC forum is really active and has a strong development on NON-official firmwares, thanks to Liggy, Dee-27 and Quikee (mainly). They allow you to speed-up your media recording (marked as 8X, recording at 16X, for example).
There are multiple threads asking for this type of comparation. I suggest you do a search and check for reviews..it's really a personal opinion.
Ah, BENQ has disc scan for PI/PO errors, something unexpected until now on future nec3500 firmwares. It's pretty good to check for the recording quality!