I'd say this season has shown that Matthew's is better than Tavares. He has been a goal scoring bandit with linemates that are sub par (geriatric Marleau, and 1st year guys like Kapannen/Johnson) vs Tavares having Marner.
I'd say Marner is arguably around the same overal value (better playmaker, not as good a scorer, and not as good a defensive asset vs Stone) but he has more 'game breaking' ability. So I see them being close in value contract wise. Some of Stone contract is buying some traditionally less productive years (32-34 are not bad, but usually after peak) but he was a UFA.
I dunno, we can speculate all day (day HFBoards!) BUT imo this contract leads me to believe Marner is closer to a sub 10m guy than being a 10+m guy on his next deal.