A BOB trailer presents a lower profile to a headwind than a set of panniers. Same with a saddle, frame, and handlebar bag combo. Both present other issues, pro and con versus panniers. Wind resistance - only relevant at high "touring" speeds (like zooming down hill) or in a strong head wind - is only one of several issues to be considered when selecting a means of carrying your kit. Most would say that carrying capacity and weight trump wind resistance.