St. Thomas is definitely one of the more “Americanized” islands - you can get really good buys on jewellery, but I would agree that the excursion to St. John is your best option. Absolutely beautiful, unspoiled and great for snorkeling.
St. Maarten is charming, but not really much to see or do. Unless they’ve changed the policy, it’s one of the islands where you are absolutely harrassed from the moment you step off the ship, with people trying to sell you everything from jewellery to hair braiding. We’ve been many Caribbean Islands and it’s definitely one of the worst for this. St. Martin, the French side, is more sophisticated and has some great restaurants and beaches - but remember that they are “bikini tops optional”.