Don't know why it's always a competition between the two but I'll play.
I think some people are confusing physical maturity with NHL readiness.
Kotkaniemi having a year of play (and excelling) against pros under his belt goes a long way.
More importantly, let's say they're both given a top 6 role with PP, I'd have no hesitation saying Jesperi would have the bigger impact. Because of one reason - he's simply a superior talent.
While Poehling could be the more physically-advanced player, Kotkaniemi possesses the more developed mental faculties, already being at an elite stage for his age. That's not to mention every other tool besides skating and strength where he's sharper.
I don't think you can necessarily confuse the two as if they're mutually exclusive. I know what you mean but I do think physical maturity plays into NHL readiness, which I'm sure you'd probably agree with.
That all being said I'm kind of splitting hairs and I know what you mean and partially agree with this. You're making a sophisticated argument. But I think that the most important aspect of a player's game or the most common attribute that needs to be developed the most signifcantly by degree is the physical side (strength, skating, etc.) Obviously skills get refined and honed at this stage but not quite to the degree that physicality does.
I guess there's also 'NHL readiness' between two players (this is the side that you're arguing I believe) in contrast with how much further along one's own path of NHL readiness is a player in comparison with another player's, which are two different things.
tldr, you made a good point which I've tried to elaborate on and failed.