I think it’s tough to say. I’m a Stars homer, but Benn did lead the league in scoring. Both, however, have been products/contributors of “super-lines”.
I’ll give the edge to Benn, to date, but I’d expect Marchand will take this vote in a year or two given the age of hockey we’re in and the stacked team that he’s on.
Let’s not forget that Jamie Benn’s physical peak was from 2011-2014 when his usual linemates were Steve Ott, Erik Nystrom and late-stage Brenden Morrow. Those early 2010’s stars teams weren’t the worst in the league because of Benn and Kari Lehtonen.