Hard not to take the play driving 1C who has proven he can produce 80+ points without an elite linemate, and is now the 1C on one of the top teams in the league. This season's production is pretty misleading due to on ice percentage variance - if Barzal enjoyed the same oiSh% as Marner (and we assume point production scaled with oiSh%), he'd have ~81 points.
Barzal's on ice shooting percentages are still way low at (7.37 at 5v5, 8.51 all situations), while Marner's are very high at 11.57 at 5v5, 13.21 all situations). So shots are going in at a 55-56% higher rate for Marner than for Barzal. Part of that is due to Marner spending time with elite finishers like Tavares and Matthews, and the other part is probably puck luck.
Beyond that, I think Barzal is just flat out more naturally talented. Marner has the better overall anticipation, but Barzal is a game breaking skater and puck carrier, is stronger on the puck, and has the better shot. Both guys have elite vision and hands, tough to pick a winner there.
This is not to take anything away from Marner who is having a terrific season and is a great player.