The difference between Phaneuf and Kessel:
.Kessel has proven he can score when it matters, PPG in the playoffs, many of you have short-term memory loss, but he looked great against boston in the playoffs last season.
Phaneuf has shown he can collapse a bit in pressure situations, sometimes he's good, other times he's horrible.
2.Kessel is a top 10 offensive producing winger in the game, this is very hard to find unless you usually have a top 5 pick in a good draft. Although admitedly, getting a guy like vanek to replace him wouldn't be a big downgrade, but the trade package to move Kessel would have to be great.
Phaneuf is a top 20-30 d-man in the league, he's not very mobile, but good positionally and strong offensively(he had an off-year this year off.), he too will be hard to replace, admitedly probably harder than replacing kessel.
both are hard to replace, but Phaneuf is not the guy to be your #1 D-man, while Kessel has shown he can be the go to guy on offense.
I dont think either has to be traded to win, but I wouldnt get mad if one does, preferably Phaneuf for a culture change.