Cousins is better than Smith for me but yeah the salary cap saving is useful. Just don't think you're ever going to win anything with Smith unless you have an incredible surrounding cast (which WAS doesn't).
Giving up a 3rd rounder is nothing considering the Skins will get a 3rd round compensatory pick for Cousins leaving in free agency so the cost there is like 20 spots in the draft.
The worst part is throwing Kendall Fuller into the deal, he was excellent for them and a real useful piece in the slot.