Dahlin, while better lately, has not lived up to his draft hype. He was hyped as having a floor of Doughty. Doughty's second season was quite a bit better than Dahlin's so far.
Dahlin is a 50/50 at least of becoming a HOFer, likely quite a bit higher. But he's not shown the generational levels so far that were touted pre draft (and by generational, the only players I consider to be generational are Hainesworth, Howe, Orr, Gretzky, Lemieux, Hasek) .
As for which one I would take, depends upon the needs of the team. A team needing offense would take Dahlin every day and twice on Sunday. He could peak at a point per game over an 82 game season