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.
I'm undecided on whether he would learn more in the AHL, but I don't find those 2 reasons all that persuasive. Although ice time is obviously very important at 20-22 minutes (Is that even normal for a F in the AHL?) you will likely have a harder time focusing on any specific thing (Outside of conditioning I suppose). Obviously too little ice time is also bad, but I would imagine the sweet spot for development would be somewhere closer to 15-17 minutes. Would he get those minutes in the NHL though, who knows, last year his ice time steadily improved over time and he was at ~15 for a few games before his injury, so it's not crazy to think he'd play this season at 15+ especially since they seem intent on having him on Dach's wing.
For confidence, I don't see that as a problem for Slaf in part because of his personality. But even if that was a concern getting cut from the NHL would be a huge blow to a person confidence, so even if success at the AHL level would buildup someone's confidence by starting in the AHL you are starting at a much lower confidence level to begin with. The team resting on someone's shoulders doesn't actually seem like it's the best learning environment as the focus would be on doing what it takes to win rather then focusing on your own personal areas of weakness. And I doubt very much there's any scientific evidence to support being "the man" is better for forming neural pathways.
I think the main advantage of going the AHL route is that currently there is an element of timidness to Slaf's game. That would be easier to work on/fix at the AHL level since it's a slower/weaker league.
But there are advantages of playing in the NHL. The best environment for development is one you dominant, it's one where you are challenged but can still do well in. As an example, one area that Slaf is supposed to be working on this year according to Nicholas is his board play. That's something that A) is developed more at practice then in playing games so ice time isn't actually a big factor, and B) would benefit from being against NHL players who are stronger/quicker/more skilled and therefore present a better challenge level for development
