I know Stastny is bigger and a better all around players than Desharnais...but I don't see how one is worth 6.5 mil AAV over 5-6 years and the other should be dumped with 3 years and 3.5 mil AAV left.
Stastny's true "value" on the cap is 5.5-5.75 mil at best. 6.5 mil(which he probably won't sign for as a UFA but maybe for Col as a hometown discount) is still a 750-1 mil overpayment. I expect him closer to 6-7 years at 6.75-7.25 mil as a UFA because teams have money with the cap going up, and the market is not strong.
Unfortunately UFA's tend to take advantage of desperate GM's hungry to sign a player to make a difference. So the price will most likely be too high on Stastny which is why it likely won't make sense. But if by some miracle he was open to numbers close to those in my post, I'd be inclined to upgrade DD.
As for DD and his contract, that deserves a thread all on it's own. He gets the numbers but in large part it's due to Patches scoring 30+ goals a year. Some will argue it's a chicken or the egg situation but I don't think it is. Patches is a consistent 30+ goal scorer which are hard to find. Yes he needs a centre to pass him the puck which DD is only too happy to do but that's not to say another talented centre couldn't pass a puck.
Given their respective linemates and past performance, I dont' see why Stastny, playing with Patches couldn't out produce DD and be a better more rounded player than DD is as well as not be pushed off the puck so easily. He's a bigger, better and with that, more expensive option but a better one none the less.
If the Habs want to get better they're going to have to upgrade on certian positons, cantre is one. Barring a trade for Jumbo Joe etc which would cost us a huge amount of future assets, Stastny is going to be as good as it gets without losing anything for him.
Worse comes to worse, there's always a market for a player like Stastny whereas DD is a much harder player to trade away.