I'll try and forecast to the end of their respective contracts:
From the perspective of playing talent, I'm calling it either a wash or a slight edge to Subban since he'll surely be in the league longer than Weber, and the better performance has gone up and down for PK while Weber's been pretty solid, despite missing a good chunk of the season this year.
From the contract perspective, the Habs are in no cap crunch and have tons of money to work with with obvious other holes to fill on the roster, but in the future who knows. PK's hit is higher and is more likely to affect Nashville in the future than the Habs being affected by Weber's, so this time it's either a wash or a slight edge to the Habs.
In terms of salary, after this season, PK has 26m left to be paid, Weber has 24m left to be paid, so either a wash or slight edge to the Habs since they can afford to eat whatever the numbers are more than most.
I think the major factor will be that while it's likely Weber finishes his career in MTL, if Nashville is able to acquire good value for Subban in the future in a trade or can re-sign him to a reasonable contract, then they can potentially forge ahead as an overall winner, despite there seeming to be no losers in this trade.
As it stands, it's pretty even, I think.