I have been to both Cayman and the Virgin Islands many, many times. I would be happy to answer any specific questions you might have. For starters:

Both are hot and quite humid in August. Virgin Islands have more winds (thus they are famous for sailing). Cayman is famous for diving.

I don't like the Westin Cayman. Just too many bad experiences. Nor do I think it is good value. This SPG Plat stays at the Sunshine Suites across the road. The beach is fantastic though.

Cayman has only had one hurricane in recent memory. It was a dandy. Most properties in Cayman now offer a "hurricane guarantee." I don't think you need to worry. VI, on the other hand, gets hit regularly. Although St. John is well positioned locationally. August is a bit early, even for VI.

St. Johns, you will be relatively confined to the Island, although you can arrange to go boating/sailing/excursions. Cayman is a well developed Island, with hundreds of excellent restarants and many things to see and do.

My advice? Try both!

Cheers,