Topsiders and Beach Blanket Buffet are essentially the same restaurant. Topsiders is on The Magic and BBB is on The Wonder, so you would not have to decide between the two.
If you meant to ask about Parrot Cay as compared to Topsiders/BBB on embarkation day, I'll answer that with my opinion.
We prefer Parrot Cay after boarding initially. It's MUCH less crowded (so moving around with carryon bags is usually less of a hassle), and I like the feel of the calmer atmosphere when I'm tired from traveling. We eat breakfast and sometimes lunch at Topsiders/BBB nearly every day of the cruise, and do so, normally, for the view of the water and the ports. PC isn't an overly attractive port at all, so I would have no reason to sit out on deck for the view.