Originally Posted by
FBinNY
The rack bag will produce lower drag than the panniers because it's mostly hiding behind your body.
But this shouldn't be a primary (or even secondary) concern for a commuter. Odds are you'll be doing most of your riding where air drag isn't that much of a factor (about 15mph).
Focus on what you need and the amount of weight you stuff onto your ride and live with any difference in air drag.
And.....to provide other over-thinking opportunities to further complicate riding a bicycle:
Take into consideration that while the rack bag is behind the butt, the panniers are not pushing through clean air themselves. They ride behind pumping legs and a whirling crankset.
Focus on what you need and the amount of weight you stuff onto your ride and live with any difference in air drag.