The difference is that a thin layer of spandex lets me tell myself that she's wearing a bra underneath it. When it's exposed skin, there's nothing to alleviate how much her breasts appear to be in fact flesh-colored hemispheres filled with helium, and that's distracting. And makes it difficult to cosplay, were anyone feeling like doing so. No amount of double-sided sticky tape is going to make my tits levitate.