Nylander is playing with 2 other rookies. No matter the talent those guys have seen about 30 NHL games combined.
Marner has a proven 60pt player and a defensively responsible 50 pt center whos good at faceoffs.
Matthews line has complimented each other (though I would like to see Soshnikov take Hyman's spot). Marner may be playing with more experienced players but he is the guy that's driving that line. Every point that comes from that line has been through the playmaking, speed and possession ability of Marner. In fact, if he played with talents equal to Nylander and Matthews, for sure Marner would have even more points now. Bozak has fanned on quite a few gimmies from Marner.
Not trying to say one is better than the other. As Leafs fan, I don't care to compare because I'm much happier to have both of them than to have to choose which one I would prefer.