Marner and Tkachuk are two different breeds. That being said, Id still go with Marner.
Tkachuk I doubt will keep up the production. Doesn't really drive the play, collects a lot of secondary points, and although he's a real good player in his own right, is certainly not dynamic or offensively talented as Marner. Though Tkachuk has other parts of his game that obviously Marner doesn't have either.
If I'm building a team and was given a choice between Marner or Tkachuk to build a team around with, most of you would already know the answer.