furballi, I have the same combination of drives as you do, and like ftp1020 suggests I use both Read Transfer tests (mostly on the NEC, but also some times on the Plextor) as well as PI-PO Q-Scan tests on the Plextor.
You should consider one thing though: It doesn't make much sense to talk about an absolute burning quality of a disc - the only thing you can measure is the reading performance of the disc in one specific drive under the current environmental conditions. The result you get is always an "accurate" representation, but only valid for that particular drive at that particular time. I have examples of PI-PO scans done of the exact same disc in the exact same PX-712A drive on two different days, that produced enormously different results (5-10 times more errors on one scan compared to another).
Don't expect your scanning results to be 100% reproducible - and certainly not from one drive to another.
If you do as ftp1020 suggests and test a disc in both your drives, you can be fairly certain that you have a good disc.