Front - fleshy painful parts, vulnerable eyes, delicate bones.
Back - mostly thick bone.
Not even a close call in my opinion.
Front - you can see it coming and possibly shift to avoid. Back - Area 17 (IIRC) in the brain - smack there and you're temporarily blind. Trust - turn your head away so your partner's knee doesn't muck up your nose during the pin - he or she is there to learn as are you. In a "real" situation with the adrenalin pumping and life on the line, you're not going to LET your "partner" pin you - it's going to be an all out scrap for survival, and (while I've never been in such a situation) I suspect you want to observe your enemy, so you can be proactive in the fight.. Trust - you're training, not fighting. You train with a partner, if it happens to be a fight, the person is not a training partner, but is an enemy.