RE: Still trying to decide...
Nothing that you shoot eill ever know the difference between the two. The WSM will have a flatter trajectory at ranges beyond 300 yards but both will result in dead animals.
A 140 or 150 grain bullet out of the .270WSM or a 150 - 160 grain bullet out of the .280 will effectively kill deer, elk, moose, or what ever you decide to hunt in the states.