Matthew Tkachuk:
CF%: 59.02%
FF%: 57.21%
CF% Rel: 9.0%
FF% Rel: 10.0%
Scoring Chances For%: 54.32%
On-Ice Shooting %: 7.32%
Mitch Marner:
CF%: 50.14%
FF%: 49.62%
CF% Rel: -0.8%
FF% Rel: 0.4%
Scoring Chances For%: 52.75%
On-Ice Shooting Percentage: 13.83%
As you can clearly see, Tkachuk has been MUCH more dominant 5v5 than Marner despite Tkachuk's most common linemates being Backlund and Frolik while Marner's most common linemates are Tavares and Hyman.
Tkachuk also only has 2 less points than Marner in the same amount of games played. While Marner has more 5v5 points, his oiSH% is almost 2x higher than Tkachuk's, which will inevitably normalize as the season goes on as the Flames are generating just as many, if not more scoring chances with Tkachuk on the ice as the Leafs are generating with Marner on the ice despite playing with an incredible talent in Tavares.
None of this even includes how much better Tkachuk is at other aspects of the game: his ability to draw penalties better than anyone in the NHL, his ability to throw players off their game with his agitation, his intangibles, etc..
People saying Marner AINEC are out to lunch. People who think Marner is better should be trying to bring evidence to the table, rather than talking without anything backing them up.