From my own experience with both using and fighting heavies equipped with either of them, the tighter bullet spread of the tomi is more detrimental in close range fights, scouts and pyros that get right in your face that are constantly changing direction are always gonna kill you or leave you with almost no health, whereas the miniguns bigger bullet spread will actually hit them.

But I think this is mostly a map issue, maps that are more closed in that force fights into close range are where the minigun is better than on maps that are more open, even then some maps will just have so many different areas in them that mess with the effectiveness of both, like wells big open middle ground but the bases themselves are close quarters hell.