I've only been to the Westin Grand Cayman -- great property and beach which I can highly recommend. Plat recognition is above average. The biggest thing to consider about Grand Cayman, though, is the cost of dining. It is NOT cheap but it is definitely world class.

I've stayed at other resorts in Punta Cana and the beach there is stunning BUT I don't have any personal experience with the Westin there. Is the Westin an all-inclusive property? If not, you might want to research dining options on/off property. Having stayed at several all-inclusive in Punta Cana over the years, I think the off property dining options are quite limited.