Both of them are poor 'central midfielders'. They don't do the basics of what you want players in that position to be doing, which ultimately boils down to getting control of the game and helping the team function. Whether it's what they do on the ball or off the ball, that control is the ultimate goal.
What Fellaini was generally very good at is when we gave him a more specific role to play, and that could be either defensive or offensive roles. There were times (especially in 15/16) we played him as a very deep DM, almost a third centre back, and he did that very well. From memory at other times we used him to pretty much man-mark an opposition player out of the match and he'd do that well. Then there were other times we used him at the other end of the pitch to cause havoc around the opposition penalty box and create space for out other attackers. If we'd used him as a squad player to fill those roles when needed I firmly believe he would have been a huge fan favourite and a great weapon to have in the squad. Instead, he spent most of his time here being played as just a run-of-the-mill central midfielder, something he was always poor at. The issue was that we kept using him there since our other options at the time were probably even worse.
McTominay is potentially quite similar. I'm tempted to say he's a little better at the general central midfield role, but not as good at the specific roles. Certainly not defensively anyway; he's potentially a weapon in the attacking areas. I won't really believe it until he does it consistently though, as he's regularly had the odd good match in the past but the bad always outweighs it significantly.
Seeing as neither of them should be playing central midfield for us, and Fellaini is better at those targeted roles, overall Fellaini is/was better.
