Love both of them. Tkachuk might be my favorite player in the league, but gun to my head I probably go with Marner if I'm trying to remain unbiased. Comparing them:
Passing/playmaking ability: Marner > Tkachuk. Tkachuk is a very clever passer just like Marner, but he's not quite the skater Marner is, which makes Mitch just a little more dangerous in that area.
Goal scoring: Tkachuk > Marner. Tkachuk just has a little more diverse toolbox there, being such a good net front presence in addition to the great wristers from the slot the two have.
Driving play: Marner > Tkachuk. Again, this is where skating ability reigns supreme. Tkachuk isn't slow and he's very strong on his edges, but Marner has another gear.
Defense: Both very good, and I'd say about even. Marner gets more takeaways, but Tkachuk is phenomenal on the boards and plays with a level of physicality that is so effective in shoving opposing players out of dangerous areas.
Both guys will be top 10 overall wingers in the league over the next decade.