DiscQuality vs. ScanDisc Tests?

For audio CDs, what is the difference between the Nero Disc Quality test (the one that shows C1/C2 errors) and the ScanDisc test? On my NEC I see occasional large C2 spikes (1 or 2 per disc) but when I run those discs through ScanDisc they look clean.

I have heard that ScanDisc gives a more reliable view for audio CDs. Is this true? What is the difference between the tests?