This is very much a matter of What have you done for me lately? Before Karlsson's surgery in 2017 I remember most people having him ahead of Coffey. But that off course was under the assumption that he wouldn't be a shell of his former self after that injury and that he would have a lot longer peak.
It's interesting to compare the two. They are fairly similar in terms of Norrises and 1st team all stars (even though many people claim Karlsson got robbed twice). However Karlsson accomplished this in much shorter time, fighting two career threatening injuries and on a bottom feeder squad in Ottawa, where he was the leader and best player, while Coffey played on dynasties where he was more a kind of backup star behind the likes of Gretzky, Lemieux, Yzerman etc. Coffey also never had the historic offensive season Karlsson had in 15/16 when he led the league in assists, 4th in points while being the top scorer on his team. He also never had the playoff run Karlsson had in 16/17 when he led a bottom feeder squad to one goal short of the Stanley cup finals. Karlsson was also by many experts considered a top 3 player in the league that year (no 1 by some) alongside Crosby and McDavid. Coffey was never considered among the leagues very best players. I've also heard that Karlsson was talked about by fans and coaches as "best in the league" in that very short stretch before his achilles injury in 2013. Karlsson was also ranked at #3 on the TSN top 50 list after the 16/17 season.
So Karlsson definitely peaked higher. The question is how much you punish him for having his prime cut short by two career threatening injuries.