We have been to St. Thomas four times (three times for a week, and one time on a cruise stop). We've visited St. John several times on excursions and I echo the comments made about the beauty of the beaches on St. John.
We really enjoy St. Thomas as well -- the shopping in Charlotte Amalie is terrific, there are great beaches on St. Thomas that appear comparable to the ones I've visited on St. John, the restaurants on St. Thomas range from excellent to funky, and for someone who is choosing to stay at the Ritz Carlton on St. Thomas, the experience is phenomenal. I would prefer to stay at the Ritz than at the Westin St. John (but would prefer to stay at Westin St. John versus Frenchman's Cove). And I would never hesitate to stay at Frenchman's Cove -- it's a great property too.
Any of the above are good options, and I think anyone visiting either island will enjoy it.
Best,
Greg