Well winning a Norris is a good start for a conversation to get started.
After that it'd be based on my viewing experience. Clearly both players are in the upper echelon of HHOF level defensemen, so the difference will come down to how they impact the game on the ice. In my eyes, I haven't seen Makar impact a game like Karlsson did, not yet. And you can say that this is because Makar plays on a great team and they don't have to run their entire offense through one guy, but that doesn't change the fact that Karlsson did it, and Makar hasn't yet. And that's what the all time great defensemen did, guys like Orr and Bourque ran their offense despite playing on great teams because they were so effective that you'd be stupid not to.
The thing that gets me is that people are saying "oh well Karlsson had the entire offense run through him and played more minutes, of course he got a higher scoring finish" like those two things are easy to do. There's a reason why so few defensemen in the history of the game have played in that situation, and that's because it is extremely hard to do that. So you can't really say "well Makar could do that too!" because playing 28 minutes a night while carrying your team's offense is something that has been done by maybe a handful of players in NHL history, so it's not really something you can just assume a player can do until they actually prove they can do it.
Crosby said it better than any one else:
Karlsson was the Ottawa Senators team. Every team's strategy was to try and make Karlsson less effective, because there weren't any other major threats on the team that could drive the play like he could. Even when he played on teams with more threats, the main focus was still to shut him down. Pietrangelo talked about it during the 2014 Olympics, the plan going into the Gold Medal game against Sweden was literally "keep Karlsson in his own end, make sure he doesn't have the puck."
THN in Sochi: Canada's mission? Shut down Erik Karlsson
Makar has a great offensively toolkit. I think he's got a better shot than Karlsson, and I think he had better lateral quickness and agility than Karlsson. I think his shot option and being able to transition from moving laterally along the blue line to quickly driving the net actually makes the superior player on the power play (Karlsson was never that great on the PP in all honesty. Better than most dmen in the league, but not as good as you'd expect. I think it's worth noting that Karlsson scored 56 EV points in 2016, whereas Makar had had 52 this year). I think Karlsson had better vision and creativity, and was more dangerous from the blueline because of how he liked to draw players out of position with his hands in tight spaces, but in terms of their play in the offensive zone, Makar might not be that far off from Karlsson. The difference as of now simply comes to how they skate with the puck. Karlsson liked to hold onto the puck a lot more than Makar, and he had the fantastic ability to slow down or speed up the pace of the game single handily. That trait is so unbelievably rare, at least to the level that Karlsson was able to do it. While Makar is a good skater, I haven't seen him control the pace of the game like Karlsson did.
So the argument that Makar doesn't do as much as Karlsson simply because he doesn't have to may be true, it doesn't really change the fact that he doesn't do as much as Karlsson. I could probably count on one hand the amount of players I've seen that controlled the game like Karlsson.
They're those rare "one-man army" players that could take on an entire team if they need to. Until Makar shows that he can carry a team like that, then I don't really see what exactly makes him better than Karlsson.