I think i'd take Duchene...but assuming we're including cap numbers here, it's a tough sell.
Duchene has the far higher ceiling. When he's on, he's a legitimate #1C. It's just...when he's not, he's not a lot of anything. Whereas Schenn at his absolute best, is a kinda #1B Center who needs some good puck controlling wingers. When Schenn isn't scoring as much, he's still far more involved though. It's the "high ceiling/low floor" vs "lower ceiling/higher floor" dynamic i guess.
$1.5M extra on the higher upside isn't a dealbreaker for me...but the maddening streakiness with Duchene, is the real stumbling block for me just outright picking him. Even though i'm kinda biased, in that i find Duchene such an immensely fun player to watch when he's actually going. I'd still do it...just really understand why others wouldn't.