For a solo dinner, I would have to say Plaza. WCC is geared more to families with children. Plaza has very nice food, and last time we ate at WCC, we were more annoyed than amused.
I picked the Plaza because if I were going on a solo trip this would definitely be a place I would eat. They have great food and ice cream. The couple of times we've been there it's been fairly quiet and very peaceful. And if you're close to the window you can gaze out and enjoy your lunch (or dinner-we've only been for lunch though) looking at the castle. We've never enjoyed WCC.
Thanks for your inputs. The night I was planning on was an EMH night at the Magic Kingdom, so I was thinking maybe it would be better to dine inside the park. I already have ADR's for the Crystal Palace & Liberty Tree Tavern.