3520a with 1.04 stock firmware - OK.
3520a with 3.04 official frimware - much better.
NEC like to release new firmwares for their drives (and there are always new hacked versions too), so the best drive today may not be the best drive once new firmwares have been applied!
Unless you want to do things with your new drive that you cannot do with your current drive (eg DVD-R dual layer) I would stick with what you have got. By all accounts, the newer drives are not significantly better than the 3500.
I don't know - I spent months agonising over whether I could justify replacing my CD-RW with a DVD-RW! You seem to want to replace a fairly new, perfectly good, DVD-RW with a very slightly better one for no obvious reason. Surely you have other things to spend your money on!