First part of the season I thought JT was just as good defensively as he was last season. Past 2 months or so though his defense has taken a nosedive.
I still take JT over Barzal if I'm playing a game tomorrow, although that could change in the relatively near future.
Edit: And I for sure think Barzal has had the better season, but that doesn't necessarily mean he's a better player.
I disagree, he was even worse at the beginning. He made a lot of real lazy, "controller disconnected" attempts at back checking. The one thing I think he's maintained throughout this year is limiting the amount of turnovers he commits. In years past, JT would over-extend himself trying to do everything on his own and often times hurt his team in doing so. This year, he has not had to do that, so he's made those plays less and less. Barzal has had issues with this from time to time, but not nearly as bad as JT at any point in his career besides this year.
JT is more refined, experienced, and plays a more obvious veteran game than Barzal. But Barzal is more explosive and dynamic offensively, more effective at creating space for his teammates, putting them in a scoring position (JT has benefitted from Barzal doing this for him often on the PP), and more involved defensively down low. He's also better at transitioning the puck from defense to offense. JT can get stuck in his own zone far too often with the rest of his team and lacks the skating ability, stick handling, and elusiveness to skate the puck out of danger like Barzal does on a regular basis.
The only thing I think JT clearly has over him at this point is goal scoring ability. You can make the argument he gets top defensive assignments, but Barzal has taken a lot of the focus away from him this year in that regard too. Barzal is quite often double and triple teamed, like JT used to be.
You're right in that just because a player has a better single season, doesn't mean he's a better player overall. But if that player is in his rookie season and the other one is in his 9th...