I'll go with 100 assists. The trio who's done it before are among the four greatest players ever (arguably top 3 even). Of the eight players who have scored 70+ goals only Gretzky and Lemieux are super elite, the rest aren't arguably even top 20 all time.
Neither Matthews nor Kucherov have eclipsed their respective round number at the time I'm writing this but if they do, I reckon Kucherov is heading for the higher all time rank of the two, which would further prove my point.
Edit: I forgot to add an opinion on the context of both feats. Not all 70 goal seasons and 100 assist seasons are equal, obviously. 38 assists is very low for a superstar 1C and supposed Hart candidate. That's not making his teammates better, which is always a huge factor of a high end player's value for their team. It's not like he's the lone offensive star of his team either.
On the same token, 32 goals for McDavid isn't that impressive either. So far good for 33rd in the league. Supposedly injured which is affecting his shooting so it's understandable. At least McDavid has displayed elite goal scoring ability with a dominant Rocket win, whereas the potential 70 goal scorer is a no show in the play making department.