I'd say you would want to have some of both on hand. In my experience they tend to do better on a varied diet. Pellets are especially good beacue you get the best nutrition for your fish all packed into a nice little package. The rest of the food, shrimp, fillets, blood worms, feeders or anything else, in my opinion just makes for happier healthier fish.
I mean that for all my fish not just my piranha.
My red bellies wouldn't take pellets so I would stuff shrimp or fillet with them. I'll do the same for my snachezi when he finally starts eating.