I don't know... I mean he WAS a non-factor for the first two and a half periods of game 7 of this year's SCF. Florida made a coaching adjustment and stuck a guy to him just as he was entering the offensive blue line, so he wasn't able to generate any speed while entering the offensive zone. At some point he made the adjustment and started accelerating in the neutral zone, but there was only about 10 minutes left in the third period when he started getting going.
If your point was, "well he required a coaching adjustment from Florida so by definition that means he was a huge factor," of course you're right but Florida was able to shut him down for the most part whenever the series was close, which I think is more important.
MacKinnon's a different beast altogether. Even if you stop him scoring, he'll still contribute physically and have an impact on the game. Obviously both these players are paid to put up points (and McD is better at that) but I think I'd rather have MacK on my team a playoff must win if I'm being honest, he's right there offensively and has other relevant ways to impact the game. Can't go wrong with either player though