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.
Couple things here:
1. Using GF/60 and GA/60 rates isn't actually a great determinant of underlying play. It's basically a glorified +/-, which we know is heavily impacted by things like sv%, shooting luck/shooting %, linemates and is volatile in general. Also, you'll notice the players with the highest GA/60 are often the team's most used d-men, as they simply play more by virtue of there being less D than forwards available and face top matchups, along with top six players as they get line matched against teams' other top players who are the biggest scoring threats. The data sample of goals is also much smaller than shots / shot attempts and scoring chances so there is a lot more noise.
Using shot attempts and scoring chances against is a more reliable marker generally speaking. In those, Marner has consistently ranked near the best for the Leafs for multiple seasons now.
2. Tage has a high CF% rel because his offensive CF impact is among the best, so overall he is still a net positive. However, when you look at just CA rel, Thompson is near the bottom, ahead of only Samuelsson, Skinner and Asplund.
In a nutshell, Tage is a high event player and there's a lot going on both ends of the ice when he's playing. However, he is so dominant offensively right now that it's outweighing his defensive shortcomings. I don't disagree that his level right now matches or even tops Marner's best, but Marner has had multiple years of consistent elite play which tips the scale for me. Will it stay that way? Time will tell. For the record, I don't think Marner is leaps and bounds better than Tage or anything like that, but a long track record will generally get the benefit of the doubt from me. You guys have a special player on your hands regardless.