Have stayed at both...Food is better in Grand Cayman (sunshine Grill across the street is awesome and fair priced). Cayman dollar is fixed at $1.25 (so just add 25% in your head). The beach is way better in Cayman (no seaweed, semi private) and if you rent a car a lot of places you can search on the island. As stated above a few nights at Punta Cana gets old. One drawback in Cayman was the bar/beach service was terrible (no service or forgot my orders). Overall the food is more expensive but I would go back to Grand Cayman especially if you can get the room on points as that is pricey if paying cash.