Other than size and maybe skating, Lehkonen was better at everything else IMO.
It is a lot more complex than that in my opinion.
Size, strength, physicality, puck protection, slap shot, shot power, shooting range and release, all of those elements favour Heineman as a prospect vs. Lehkonen from before the 2016-2017 NHL season.
Skating I would put as fairly equal overall, as I think Lehkonen had better edges and agility though lesser top speed and weaker balance in comparison to Heineman.
Defensive play I'd rate as roughly even too between the two. Lehkonen was maybe a bit more polished at the same age defensively but Heineman has an easier time displacing the puck when battling opposing forwards/d-men on the boards, or clearing it when pressured because of his strength advantage so that levels things a lot.
Hockey IQ I would give to Lehkonen slightly, but Heineman is no slouch there either.
Puck-handling and passing I would definitely give to Lehkonen, and that explains a good part of why he had more success in the SHL than Heineman before going to the NHL.
Then we start noting their styles of play, and comparisons become a lot more sketchier between the two, as their playstyles are actually quite different.
Lehkonen as a prospect was a good two-way high-energy puck-hound with speed, smarts, an overall good shot and skills to flash at times. He'd do the majority of his work as a (very) positive puck-possession player, relentlessly put pressure on the puck carrier, force turnovers and then use his skill/speed/smarts to try and create something in the bigger ice surface of the SHL, be it a shooting or passing opportunity.
At the NHL level Lehkonen did much of the same thing, but had to of course adapt some of his game to fit the smaller ice, learn to take quicker decisions and simplify his game.
Heineman is also a good two-way player with high-energy, speed and smarts, although less skilled than Lehkonen, and that's where the comparisons come from I guess? But he is much less of a puck carrier than Lehkonen, preferring to defer to his teammates when it comes to carrying the puck/making plays, and working more as a "facilitator" on his line (as opposed to Lehkonen's buzzsaw/puckhound style) creating space with physical play and then waiting to get fed pucks to snipe or screening the goalie.
Coming over from the SHL to the AHL, Heineman also has had to adapt. But the details to his game, how he prefers give-and-go, quick touches on the puck as he looks for good shooting positions while also supporting his teammates in any way he can (cycle, boardplay), fit the NA ice and schemes like a glove.
So yeah, we have two very different players here, with playing style differences galore and some amount of similarities too, all of which make comparisons fairly moot.
Just putting it out there, I think that with some positive development in the next couple of years Heineman could thrive in the NHL as a middle-6 two-way forward with speed, physicality and good overall sniping abilities.