The answer is not as obvious as you may think.
If he'd asked about full power loads, then it's a slam dunk for the 300, and that is where it's real advantage is. But in subsonics - the choices of effective subsonic 30 caliber bullets is very limited, and even harder to find in loaded ammo. In 9mm on the other hand, we've got a lot of very good choices that perform a lot better than the typical subsonic 300 Blk.
Too many people get hung up on the idea that somehow a subsonic round is best for their home defense, without understanding the limitations. Often the same people claim to be concerned about overpenetration, without understanding that non-expanding subsonic 300 Blk (or any 30 caliber) penetrates far more than a decent expanding pistol bullet.