The ball, because then you press with your toes, which uses you calves, and takes some of the strain off your thighs and knees.
You can get plastic clips that screw on the front of the pedals. Your toe just slips into them. They hold your foot in the right place,mand stop your foot slipping off the pedals if you hit a bump.
You can also get "bindings" where you have a clip in the bottom of your shoe that fixes on to the pedal, but I don't like them. I don't like being fastened to the pedals too much.
But toe clips really do help.