Debatable. Marner has a slightly better xGA/60 this season but Nylander rates better in CA/60, SCA/60, and HDCA/60. Matthews has also been very good defensively this year and they also have Hyman on the line who we all know is a defensive stud. Nylander has Tavares who looks a step slower this year and a revolving door for LW.
They've kinda alternated in the previous seasons... Marner was one of the worst defensively last year while Nylander was about average, but Nylander was more sheltered due to coming back mid-season and struggling. In 17-18, Marner was better, but he was also more sheltered playing lower in the lineup for the first half of the season or so until he joined Kadri's line permanently.
As rookies, Nylander's numbers were much better. Over the 4 cumulative seasons, Nylander ends up rating better overall, but they've obviously both grown as players. Nylander also faced harder competition his first two seasons than Marner did.
IMO Nylander has better positioning and natural defensive awareness, and is also less prone to turnovers while being stronger on the puck and boards. Marner plays better 'active' defense and is good at forcing turnovers from the other team.