Now and future: Rielly
Rielly is approximately the 20th best dman in the league (low end #1). He looks poised to bump himself higher up the list this year with his red hot start. His defensive play is significantly better than it was a couple years ago as his rush defense and decision making (pinching in or not, skating in deep, pass/dump in, etc.) is great. He just needs to improce d-zone coverage a bit more (which he has been working on).
Chabot on the other hand looks like the real deal. He could be a Norris candidate down the road, but defensemen develop weirdly and a ton of great young dmen never continue to develop as expected. Loved his play in the world juniors and think he should be at least a valuable piece moving forward for the sens.
Chabot just hasnt shown enough yet at the NHL level to be in a poll against Rielly. Dmen are even harder to predict how they transition from pre-NHL to NHL play than forwards so it makes it even tougher to gauge how good he will be in the future since hes only played 1 season so far.