Yes, you do get more attack and health to your creature with Chaos Power for the same price - but one would rarely use Unstoppable for that reason (except for certain OTK decks).
You use it for one very specific reason: shield bypass. It does not matter how big and strong your creature is if it cannot hit due to a shield (dim, grav, proc)
Also, the other main difference between the two cards is that with Unstoppable, you know peerfectly what you will get from it in advance. With Chaos Power (or Seed), you have no idea, and therefore cannot be sure if your plan will work or not.
Hence, a 50% cost reduction is enough. Sure, it could use more buff, but it is not necessary