I feel like you might bleed out a bit faster that way. It takes a minute to get through the skull, the pelvis has all sorts of joints for the saw to get through and if they're taking the spine out with the cuts you might not feel a ton after a few strokes.
Nah, head would be the way to go. You would bleed out so fast and there are not a lot of pain sensors there. You would feel a bad headache and then the saw would tear up your brain, and that’s it.
Pelvis you will go into a long state of shock, but only after your genitals are all cut up.