It's tough to compare the two really. Due to style of play, but especially due to "supporting cast", which kind of compounds the stylistic differences. Heatley was a tremendous goal-scorer, mostly a particular type of goal too. Very narrow skillset, but very focused and extremely good in that context. He was always sort of just the "finisher" even at his peak...and leaned a lot on a pair of other outstanding players with skillsets that really filled in where he was lacking.
Kaprizov on the other hand, is kind of an "everything" player. He does it all, and he does it largely by himself. He's capable of elevating other players around him in a way that i'm not sure Heatley ever was. I mean...he turned Ryan Hartman into a 34G - 65Pts player for cripes sake. I love the way the guy plays, but he is not actually nearly that good, and there isn't a chance in the world that Heatley would've been able to lift a linemate to that extent. And it's not like Kaprizov can't score all sorts of goals himself too.
So for me, it's Kaprizov. Though Heatley on that Pizza Line was bigtime, and his consistency in scoring goals was pretty impressive through his prime. So i wouldn't really rail to hard on anyone who votes that way. I just think the ability to elevate linemates tips the scales for Kaprizov.