What a great question. I have a lot of experience in hunting and testing of both mechanical and fixed broadheads and I don’t even have a clear answer for that question. There are times that a mech is the way to go: Like, if you have an untuned bow or are going to be shooting very long distances or in heavy winds.
Mechs tend to be more forgiving in flight. Also, mechs are a great choice for smaller animals, like turkeys, hogs under 150#, black bear, and smaller deer. As long as your bow is generating sufficient Kinetic Energy, they can do very well. But for larger hogs, larger deer, elk, moose, larger bear, etc, or if your bow is not producing high KE, I would suggest a very strong, deep penetrating fixed blade head. If I were to only have one style of head, it would be a fixed blade.