If Cousins wants to be in Portland, I like Cousins a lot more than Whiteside (though I like Whiteside too). Cousins is a better two-way player--he's not the defensive wrecking crew that Whiteside is but, in the periods where he's put in effort, he's been a good defensive player and the Kings have been significantly better defensively with him on the court than with him off.
Of course, if Cousins doesn't have any interest in being a Trail Blazer, I wouldn't be interested. An engaged Cousins can compete with players like Davis and KAT for best big man in the league. A petulant, checked-out Cousins is just importing the Kings' biggest problem. So, the Blazers would need to get permission to talk to Cousins before any deal, to gauge his interest, in my opinion.
An enthusiastic Cousins is, to me, worth a lot more than McCollum. He impacts the game more ways and can actually be a building block to a much better defense.