Good poll.
Dermott you have a high offensive IQ shifty puck moving defenseman that is already comfortable being the lead man to exit puck out of zone and commence breakout. His responsibilities continue to increase and he has quality top 4 (fringe top pairing) potential. He looks like a carbon copy of Jake Gardiner and I'm impressed at how he has made his skating an asset at this level. His hands are pretty good too.
Carlo you have a high defensive IQ big smooth skating defenseman that is comfortable making the first pass, has long range and looks to prevent zone entries well, is already comfortable playing big minutes against solid competition and while has had his ruts, is doing so while playing tough minutes on one of the league's best teams.
I think who you pick is based on team need. What Dermott will provide you is valuable if you need a transition puck mover, who can make plays in neutral zone with speed, and there is value in a 6'5 defenseman who can skate that eventually will shut down the elite forwards of the game. He'll eventually become an elite shot suppressor, who will play PK minutes and if he continues to show that he can make good smart passes out of his zone to transition the offense, won't need to be a 40+ point defenseman to be valuable or first pairing.
I'd pick Carlo because the Leafs need a big right handed defenseman that can keep up with the play and shot suppress because we don't have that. Dermott theoretically has a higher ceiling probably because he has shown in limited minutes to be sound defensively, is aggressive and his offense is higher, but for the Leafs and our window to win a Stanley Cup I wouldn't mind having a Rielly - Carlo pairing to ease the tough minutes that Rielly plays.