With all characters, its entirely dependent upon which character has trained themselves more. A hollow on has the potential to be more powerful than any shinigami could ever be(Vasto Lord stronger than Captains). The difference with a vizard and an arrancar is that the barrier that limits the power either race could achieve is dissolved. They both gain each others powers inorder to advance to the greatest power achievable. In an easy manner to understand, we could give numbers. The power of a Shinigami could never go higher than 12,000. The power of a hollow could never go higher than 15,000. Becoming an arrancar or a vizard requires gaining the powers of the opposite raise. So shinigami who adds hollow to their power (12,000+15,000) has now increased their maximum possible power to 27,000