I prefer an Osprey medium sized messenger bag worn low so the stabilizer strap fits around hips like a belt, and the bag sits at the back of the hips. For commuting, it carries a roll of clothes, food, and basic tools (approx 10 lbs total).
It's low enough to not affect the center of gravity much, allows more air to the back, and it fairly aero in the drops.

I used the same bag on solo century rides last week -- one in the mountains. No problem.

Like gregf83, the weekend rides (40 - 70 mi) feel fabulous when I leave it home.