I've never been to St.Thomas so can’t provide any comparisons but have vacationed on SXM more than 20 times, so we obviously enjoy the island.
Canuck Travlr gave you a good overview. If you’re planning to exchange for a timeshare, you'd likely be staying on the Dutch side. We own at Divi Little Bay, and love the beach and facilities there. But there are other timeshares on different parts of the island that have their fans as well. We have also rented VRBOs to stay on the French side for a week along with our week at Divi.
What we like best about the island:
great beaches around the island so you can easily explore and enjoy different settings. The beautiful blue water of the Caribbean, lovely sand beaches, and beach bars that range from casual to gourmet. In addition to Divi, we usually visit Orient, Friars and Mullet beaches.
amazing food, especially on the French side in Grand Case and Orient Village, but also many excellent choices on the Dutch side, too. Food comes in by plane from France, the US and many other locations and the quality is top notch. It's not a place where you should do all inclusive…too many excellent places to try if you enjoy fine dining. I can share some favorites if you let me know what kind of dining you like.
friendly people, especially at restaurants and resorts when you've visited a couple of times. But in general, there's a friendly vibe around the island.
What we don't like:
Over development in some parts of the Dutch side. Politicians seem more interested in lining their pockets than protecting the environment and the beauty for the future.
Traffic can be frustrating, especially on days with multiple big cruise ships in port. You’re probably best with a rental car but some people find a taxi driver they like and use that person or others use the local minibuses that run around the island. Way too many scooters driven by guys with a death wish can make driving stressful sometimes. But it’s easy to navigate thr island because it’s pretty small and you can easily go across the island for a beach day or restaurant.
Things to do besides beaches:
Small boat excursions to the neighboring island of Anguilla, also known for its amazing beaches or for going around SXM with snorkeling. Not great snorkeling but there are a few places to visit. There are also some sunset cruises.
Ferries to nearby islands of Saba, St. Barts and Anguilla. I've not personally done this but many people do.
Rhino riders excursion, a fun outing where you go as a group and drive a small “boat” that's like a jet ski enclosed in an inner tube. Fun way to see all the huge yachts wintering in the lagoon.
Parrotville, an aviary space filled with all kinds of exotic birds that are eager to land on your hand to enjoy the cup of food you're given
Shopping especially if you’re interested in fine jewelry. Also shops with higher end brand clothing and some stores on the French side with unique items. But there's also a market with some crafts people in Marigot, the capitol of the French side on Wednesday and Saturday mornings.
Bridge opening. Several times in the morning and afternoon the drawbridge opens that allows many yachts to enter or exit the lagoon. You can have breakfast or a drink at casual yacht club bar right beside the passage way and watch the action—it’s a tight fit for many of the big yachts. (There's a video on line you can probably find where one of the boats actually knocked off the bridge control booth, with the operator managing to barely jump in time.)
Enjoy sand gravity. That's the feeling you get when you relax and just enjoy the island, sit on the beach, and decide you really don't need to do all the things on your list!