I think Slaf’s ceiling is a ~40g, ~40a type power winger, but I don’t think it’s all that likely he reaches that potential, maybe a 20% chance, if I had to put a number on it. His floor is a bottom-six guy, imo. Most likely scenario for him imo is a top-six guy who’s good for about ~30, ~30a in his best few seasons. Most likely an underwhelming #1 pick, but everyone knew that going in - it was a weak draft.
Lafreniere looks like a career 3rd line guy, most likely, maybe a couple seasons of average 2nd line production. Worst case he pretty much stays what he is now. Small chance he has some kind of sudden upward curve in development, something similar to Sean Couturier - basically the least likely scenario that’s unlikely to happen, but it could.
I’d pick Slaf if I had to choose between the two, and not because I’m a Habs fan. Laf just kind of sucks, and in all honesty, so has Slaf in the little we’ve seen of him so far. The difference is that Slaf is 18 and very raw, whereas Lafreniere was touted as NHL ready from the draft and has been very underwhelming and hasn’t progressed much 3 years in.