I'm going with the back pack, easier to handle, if you put it on the bike and need to throw the bike around a little for some reason, it's going to be harder to maneuver and will mess with the fore aft balance of the bike depending on how you secure it..
a man's heaviest part is around his chest anyway so the extra weight there would not be as bad to maneuverability. a woman on the other hand is usually hip heavy which may be a reason to put the weight on the bike.