I find backpack strapped onto my back better for "aggressive" riding. Yes, higher weight and moves as I move, but I'm the one controling it. When it's on the rear rack, the bike beneath me is heavier, less responsive somehow.

Having said that, for most casual riding, I much prefer the backpack strapped to the rear rack - always do it that way. It's a lot more comfortable and it is a bit more aero.