Well--neither HAS to be dressy--unless YOU want to dress up. The only prohibited dress at either is no bathing suits and no tank tops on men. BUt for goodness sake please don't think you'll need a coat and tie, or a dress and heelsl!!!
In a general way, I've seen more folks "dressed up" at CItrico's. It's a bit more upscale than Narcoosee's, more quiet and fewer kids.
For what it might be worth, we always wear nice walking shorts to both places and it's always been just fine.