I tend to look more at experience than age. They were both rookies and Barzal showed he could dominate the competition, Hischier did not. Obviously age is a factor but they're 1.5 years apart, it's not like Barzal was Panarin playing in a top pro league and put up big numbers as an older rookie.
Sure, Barzal had easier matchups but he didn't get to play with this year's Hart winner. Can you honestly say with a straight face that Hischier would have put up better numbers playing on the second line than he did with Hall?
Of course development isn't linear but why should we assume that Barzal just had a career year as a rookie while Hischier will double his production. I'm basing my statement on what has already happened and working from there. It seems like you're basing yours off of Barzal hitting his peak at 20 while Hischier was just scratching the surface at 19.
Hischier is definitely more likely to increase his production because it's a lot easier to break a 52 point career high than it is to break an 85 point career high. Barzal might have tougher matchups but he's also going to have better linemates and he's now "the guy" for the Islanders so everything is going to go through him. Hischier will almost certainly improve but he's also a valuable defensive forward so I just don't see him getting the matchups and scoring minutes to break 100.
I'm not even doubting that Hischier could be the better overall player when all is said and done. I just think its much more likely that Barzal hits 100 than Hischier.
Although they were both rookies, a year and a half of extra physical maturity definitely doesn't hurt. It was also only Hischier's second season playing in NA too.
Other than that, I pretty much agree with everything else .