I guess it would depend on whether I wanted the character meal at CP. We've had dinner there twice and enjoyed it, and our kids love Pooh and friends. My husband and I ate at the Plaza by ourselves last year and it was good, but it's just sandwiches and salads. To me, you can't really compare CP and Plaza because they are different experiences.
I will say that if you're going for
free dining (done it twice) I'd pick the CP dinner over Plaza. More bang for the buck and great variety.
Hope this helps!