Anybody who thinks Dahlin isn't very good, even just now, doesn't have an opinion worth considering. They either can't actually evalutate hockey ability, or are so coloured by their own fandom that they refuse to accept that there are other good players in the league that aren't on their favourite team. Either way, best not to take offense.
I don't really care to get into this debate, but I know I wouldn't trade EP for Dahlin, despite the fact that he very well could end up the best defenseman on the planet. My comparison for Petersson has been Malkin since draft day, and now that now that the comparison seems like it could have some legs on a production scale as opposed to just the stylistic, I couldn't imagine giving him up.
At the end of the day, if Pettersson ends up as Malkin-esque and Dahlin ends up as Doughty+, I think you could very easily argue it either way. Depends on team needs and preference, but you couldn't go wrong with either. All I know is that Sweden is going to be a pain in the ass to play against for the next 15 years.