I think it is a win-win for both side. Both players are franchise defenseman and both bring different qualities. I don't think Subban is better than Weber, nor i don't think Weber is better than Subban. They are different player who are both franchise defenseman.
Now.
Subban is a spectacular defenseman who has a unique toolset. I don't believe that Montreal was shopping him. I also don't believe there was an unsurmontable rift between PK and the rest of the team, that's media drama. I believe that if it is not an offer of Shea Weber, Subban is still a Montreal Canadiens for the next decade.
Subban also has a particular personality. He is not really the leader you want in a true #1D (Compared to Keith or Doughty, for example). I think he will thrive in Nashville because he will play with guy like Josi, Ellis and Ekholm. He will only improve their D and will complement them with his flashy style. He will not have to be their #1 go to guy on defense. He will be able to be himself totally. Subban has not the quality you research in a #1 defenseman when it comes to team play and leadership.
I'd also like to point out that leadership in Montreal was weak last season. As much as Subban was not the problem, he was not the solution and didn't had a season you would have expected from a true leader. (Also True for every ****ing player in Montreal).
Concerning Shea Weber, you have another franchise defenseman, who has a long, big and tough contract and who is due 8M july 1. (Couple of days after re-signing Forsberg 6x6).
Shea Weber is the perfect prototype of what you want in a true number one defenseman. Big time Leadership. Can play 30 min a night. Big. Tough. Big Slapper. Moves the puck very well. I feel that his impact is undervalued because he was part of the (still) great Nashville D.
Shea Weber will have a much stronger impact on the Habs than PK, as much as PK will probably be a better player in Nashville than he was in Montreal.
This being said, as a habs fan, i just don't see how our team is weaker today than it was two days before. And yes, i've seen everything i had to see from Shea Weber.
I think there was one player in the league that could pry Weber from Nashville and it was PK. (Favorite of Poile, All-star game in Nashville, better fit in terms of contracts, Nashville got younger and speedier, will take on Weber minutes.)
I also think there were not a lot of players that would truly make Bergevin interested in trading PK and Weber was surely one of them.
I see a win-win for both teams as Nashville improves their situation and Montreal get the prototypical number one defenseman Shea ****ing Weber.