As a Flyers fan Schenn will probably be the number two center with an 80-90 point peak while Couturier will be the number 3 with around a 70 point peak. I'd take Schenn, but it's close.
As an NHL fan, your expectations are absurd. When's the last time a team had 3 centers (who were all actually playing center) who all put up 70+ points in the same season? It basically never happens, because it's not possible to have 3 different centers playing major offensive roles, getting huge minutes in offensive situations and on the PP. I can certainly see Schenn and Couturier both having 70+ point upside, but only on different teams or if moved to the wing. Also, 90 points seems pretty ridiculous for Brayden Schenn, regardless of his situation.
Re: the original question, I'd go with Couturier by a small margin. I think Schenn has a bit more offensive talent, and is definitely the faster skater, but Couturier is a lot bigger (well, taller, as a teenager he still has to fill out), and a lot better defensively. Both are IMO gonna be "good #2 C" type talents, but I'd give the edge to Couturier for his defensive game, a center with great size, with the ability to kill penalties and play shutdown, while also contributing at a fairly high level offensively, that's a pretty rare thing. Guys like Jordan Staal and Patrice Bergeron are invaluable, especially in the playoffs, and I can see Couturier playing a similar role.
The sample size hasn't been large, but I have seen enough to get the impression that Couturiers ceiling is higher than Schenn. At the present time, Couturier's game is far more polished. Schenn is a "run around" type of player. The only reason Schenn gets the offensive role is because Couturier has the hockey IQ to succeed in any role.