St. Maarten is a beautiful island. Maho beach is a must see for first time visitors. It's an entirely surreal experience to hang out on the beach or in the water and have massive commercial jets pass over you and land feet away from you. Be prepared to be inundated with requests for cabs. There isn't much to do or see at the initial port. You can do a 15-20 minute walk to see more, but I highly recommend taking a cab to your destination. All of the best parts of the island are 20-40 minutes away via cab rides, so take that into consideration when traveling around.
St. Thomas has the most beautiful water and landscape, however is the most tourist heavy island I've been too. I don't mean lots of tourists, I mean lots of sketchy tourist attractions. The butterfly exhibit is run down and depressing. Of course there's a Senior Frogs, as there is everywhere. The ocean is gorgeous as are the beaches. We opted to walk around the beaches and explore things ourselves, the only negative was that I got tired of the locals attempting to sell me various illicit materials. The skyride is sketchy. Beautiful views, but in dire need of some maintenance.
If you've ever been to Nassau, you realize very quickly that it's equivalent to a highway rest stop. Getting through the port is a nightmare and the island is just not anything to write home about. You have Atlantis, but you have to work hard to get there, Atlantis is in no way near the port. Give yourself at least 25-30 minutes to get there, if you're lucky. It took us an hour to get there due to the congestion at the port, it did only take 45 minutes to get back.