From: United Kingdom
If Doctor is fussier at high speed, but does NOT show a raised error count on good media, then high speed is clearly the way to go - a faster and simultaneously more critical test.
CDspeed seems to vary - it can show yellow for a sector, on a CD that showed no C2 and acceptable C1 in Doctor.
The proviso to using CD doctor in FAST mode, is that it does manage to show an acceptably low error rate for CDs that are good.
One final note for CDspeed - with some drives, it seems you need to start and than abort a transfer rate test - and then start the quality check very quickly, in order to run that at high speed.