The LiteOn 1635S/SX should be very good for "testing" DVD media if what you want to do is run Disc Quality scans with Nero CD-DVD Speed or KProbe.
If you want to scan CD media however, you should avoid LiteOn DVD burners because they don't give meaningful results. Then a NEC DVD burner is a better choice, but NEC DVD burners are not the best for scanning DVDs.
A BenQ DW1655/1650 is good for scanning DVDs that are burned in BenQ drives, but there has been some debate about whether to rely on BenQ scans for DVDs that were burned in non-BenQ drives.
You can also scan CDs with a BenQ 1655/1650 and it will report C1 and jitter in a useful way, but it's probably too optimistic about reporting C2 errors.
I hope that didn't just confuse you further.