I think of them both as techniques for the same purpose - to get you to draw what you see as there rather than what you know is there.  (Or even drawing the symbol for what you know is there.)<p>It's been a while since I've done any drawing, but from what I remember you can easily use both in the same image - blocking out the constituent shapes with approximations with around the right proportions, and then making sure that the negative space looks about right in the gaps in between.