Which is better, I just can't decide!
For first person shooters, the mouse and keyboard are always better because you can look around faster, aim better, etc.
Since you specified Fortnite in your tags, I assume that you relate the question to fortnirlte and M&K is clearly better there, take a look at the pro scene and look for controller players, is only the smallest part.
I rarely play Fortnite. If so, then with the mouse and keyboard.