Under the current system, where they spend "hours" messing about drawing lines down from the armpit/nose/backside/knee/chest to the ground, on a 2D screen representation of the situation being decided upon, I do not believe for one minute that they know exactly how far downwards to draw the vertical line so it intersects with the pitch, in order to provide a fair comparison with a body part that is at pitch level. The line from Peter Crouch's armpit is going to be a lot longer than needs to be drawn for Dennis Wise....its guesswork at best.
For that reason alone the measurement criteria should be the position of the feet touching the ground.
Also, one of the biggest battles on the pitch is the timing of a strikers run and a defender "stepping up". It is perfectly feasible when a striker to be running to wards goal that his natural body lean may mean his upper torso is goalside of the defender, whilst his feet are "well onside"...my personal view is that that is a well timed run, and should be rewarded.
Just my opinion.
This. With bells on.