Personally I wouldn't worry about it a lot.
Depending on how you have your bike, in the first photo, the seat and back would be mostly above the bag anyway, but your thighs still have some bulk to break the wind.
I'd get whatever is most convenient for whatever you're carrying. A few extra pockets are nice.
On the other hand, big open space is nice.
I carry two duffels that are bigger than that on my Cargo bike. They are a little awkward, and I'll probably do panniers sometime. But, I can put a full 50 pound bag of flour into each duffel bag.
The Duffel bags give you a central load. The panniers give you a low load. Each has its benefits.
As far as going faster...
Consider filling your bags with rocks at the top of a mountain and emptying them at the bottom.