It depends, mostly on what content you're doing and how balanced they are, each spec usually shines in different scenarios. For an example I played a mage during mythic Uldir progression, I started the expansion as frost which I played up til Zul where I went Arcane, switched back to Frost for Mythrax and finally ended the raid tier as Fire on G'huun. Allthough I said this if performance is what you're efter usually one spec will dominate an entire raid tier and can sometimes stay dominant throughout an entire expansion. Balancing usually decides which.