Been to both. Punta Cana is easier, cheaper, and fine if you just want to sit on a resort and drink. But you said "snorkeling." Curacao wins that category by a mile. The snorkeling right off the shore at Playa Lagun or Tugboat Beach is world-class. Punta Cana snorkeling is mostly choppy and requires a boat trip. That said, $600 is $600. I'd save the money and do PC, then plan a full week for Curacao later.