Heiskanen, by a lot. I'm a Jackets fan and Heiskanen is easily better defensively compared to rookie Werenski.
Werenski in his rookie year was getting caught on pinches multiple times per game. That or getting straight up burned by faster players. We had a lot of debates about it on the Jackets board at the time. Werenski never got much of a bad rap for his defense because Jones, Savard, and especially Bobrovsky were incredible that season at covering up for him. Bobrovksy probably faced a dozen breakaways that year purely on Zach's account. I seem to remember him stopping all of them. He's just now, a couple seasons later, becoming a top pair level defender. Heiskanen, in my limited viewings, is at that point two years earlier.