I think that Slafkovsky would learn more in the AHL for a few reasons.
The most obvious reason is the increased ice time. He would hopefully be given 20-22 minutes a night which is more than he'd get in Montreal, and that would include situations that he would not see in Montreal such as 5on4 ice time, 6on5 and 5on6 in the closing minutes of the game, and 4on4 in overtime. That's valid education. I actually think that if he's in Laval he should get some PK time as well. Guaranteed he won't get PK time in Montreal.
The more important thing (I think) relates to confidence. He would get to be "the man" in Laval. The team would rest on his shoulders, so that's better for learning, for forming neural pathways, etc. Whereas in Montreal he will learn to be a sidekick, and possibly be worried about making mistakes.
A line with Lias Andersson and Joshua Roy with defensive help from Logan Mailloux and Jordan Struble would provide wonderful opportunities for learning all facets of the offensive game.