Hintz doesn't have Janmark's agility - the thing that makes Janmark a somewhat useful checker. Right now Hintz is in the NHL because his awareness makes him promising, but he hasn't been able to use that to his advantage at this level yet. Stylistically I think Hintz plays the game like a poor man's Scheifele. There's a pretty big difference in talent level and Scheifele as a teenager was still stronger than Hintz is now, but in terms of the way they want to create offense I see a lot of similarities.
If you asked me to take one player for a single game right now, Janmark is a pretty easy choice because he's just a more useful player, even if his use is somewhat limited. But Hintz is still a rookie, that's to be expected.