Playing in the NHL with the opportunity to immediately move up the lineup when someone inevitibly struggles or gets hurt, building a rapport with the coach, facing NHL defensemen, killing NHL penalties, playing off the ice with elite skilled teammates and growing from that...eventually contending for the Calder Trophy with your immense talent.
vs
Playing in the AHL with the opportunity to get a callup for spot minutes while the coach treats you like a hand-grenade AHLer who's never played in the league, facing AHL defensemen as everyone handwaves your success as being against AHLers, killing AHL penalties, playing off the ice with the likes of Luke Gazdic as the "team leaders" and on the ice with prospects who've about played as many or less NHL games as you have while approaching your waive eligibility status with minimal NHL GP and generally being on bust trajectory in the eyes of most, which in turn directly affects the beginning of your season the year after.
It's pretty much the difference between how you treat a Mark Stone and an Anton Lander. Is Jankowski an Anton Lander? That's what you've decided if you send him down. Jankowski is a Mark Stone. He should be in the league that accelerates his ascension to an excellent hockey player.