This one is tough. Both can win battles simply by being pushed into battle, including stopping chump blocks.
I went with Thad for this reason: If your opponent is playing a light defense, he probably won't be able to stop either one, even if Thad doesn't have his full posse in play. However, if your opponent has a decent defense, he might be able to stop John with a negate, a CBIg card, or a battle extender like UW or UN. Thad can still be invincible, no matter the size of your opponent's D.