I'd have to say the bag is 100 times better than those punchmen.

Initially the punch man offers no movement, it is a stationary target. This is detrimental in two ways,
1) it is hard to set your punches and get in the correct position.
2)Your own movement will become neglected.

I feel the bag is also a better tool as you can pick thge area you want to hit on and you will get a specific reaction from each puch you throw.