Regular season, they are arguably even.
Playoffs, I have not seen Matthews take over like Mackinnon. Mackinnon is a 1.38pts per game player in the playoffs, Matthews is a 0.75pt per game player... Mackinnon has led his team or been 1 point off the lead in playoff scoring every year he's made the playoffs... Matthews has had "good" series and is normally at or near the lead, but also had a playoff where he did absolutely nothing. Additionally, Mackinnon has actually gone on runs.
So... almost 2x as many games per playoff appearance, almost 2x more productive per game offensively, 17 pt team lead in playoff scoring since entering the league vs Matthews being 2nd, 1 pt behind Marner... how on earth are their playoff records not that different?!?
That said, Matthews belongs in that conversation and will continue to entrench himself within that top group more and more and more. But acting like there is no justification for him being considered a step below Mackinnon, while acting like they have been comparable in the playoffs... well, you're trying to bullshit us.