I've watched Sabres games, Tage is a great player but please don't try to convince me that he is some defensive stalwart. The numbers are what they are, the Sabres give up more chances and shots when Tage is on the ice, and he plays a high event style in general. Nothing to be upset about, he will hopefully learn over time.
Tage's defensive play isn't amazing, but it's also being undersold. The team as a whole plays with less defensive structure than Toronto and with how they've been playing thus far with much worse goaltending. Even with worse goaltending Tage's numbers aren't that far off from Marner's.
Additionally Tage's CF% Rel is 2nd on the team at 5v5, which tells me as a percentage they are not giving up more chances with Tage on the ice. In total perhaps because he logs more minutes, but by that same token so does Marner.
Tage's GA/60 stats
5v5 - 3.2 (5 forwards worse without counting Sheahan because he only played 2 games)
PK - 6.7 (8 forwards worse) Tage get's a little over a minute a game on the PK
Marner's GA/60 stats
5v5 - 2.7 (2 forwards worse on the team)
PK - 7.9 (2 forwards worse on the team) He gets a bit over 2 minutes a game on the PK
Tage's GF/60
5v5 - 4.8 (2nd among Sabres forwards)
PP - 14.8 (1st among Sabres forwards)
Marner's GF/60
5v5 - 3.0 (5th among Toronto forwards)
PP - 11.2 (2nd among Toronto forwards)
Now I'm not saying Tage has as good of a track record as Marner, or that he is as good defensively. What I will say however is that Tage right now offensively is playing better than Marner currently, and probably as well as Marner ever has. Going forward If he continues to play this well the gap between them is not as big as it's being made out to be. And just to point out I voted for Marner when this poll was made.