In OSX a dual 1Ghz QS should be faster then a single MDD 1.25ghz system. You should be able to get a dual CPU for the MDD cheap on ebay. As far as selling the QS, they are not worth much these days so keep it as a spare, MDD seem to blow up their power supply for some reason.

I own a couple QS units (867, 1.25Ghz Sonnet upgraded one) and since I need OS9 native booting machines they are probably the last powermac I will be adding to my collection.