Tkachuk isn't lacking in the puck skills department, and definitely has the better shot over Marner. Tkachuk should hit 40 goals this year.
Tkachuk is actually a very underrated passer and is typically the guy setting up his teammates on his line with Stutzle and Giroux. Sure, Tkachuk might not have the "dipsy-doodles" that Marner demonstrates, but I find those are more flash than actual results with Marner, and he routinely flubs them when it comes to actually burying the puck. Tkachuk isn't lacking in the ability to deke someone out of their joke strap either mind you.
Marner plays an extra two minutes per game over Tkachuk, which definitely helps with Marner's point totals. All the while too Marner has been playing on a (much) stronger team over the course of their respective careers, so I'm not putting that much weight into the discrepancy between their point totals, at least, over the last couple of years.
Is Marner more creative? Sure, but I don't know how much it actually benefits his effectiveness. It sure feels like he is a little TOO creative out there half the time, and that it impacts his chemistry with his linemates.