Welcome to the USVI

Both STT and STJ are very different from SXM.
I will not compare and contrast and try to stay focused on question at hand.

Both islands have a nightlife, but the day time is what is all about.
ST Thomas (STT) is more active after the sun goes down, but no casinos etc..

Snorkeling off the beach is amazing so I hope you guys are swimmers.

There are no umbrellas, chairs, or beach bars on the beaches, especially STJ, because the beaches are shady naturally with local trees and bushes and part of the National Park. Most villas provide chairs and coolers.

Highly recommend a day trip over to Jost Van Dyke. It can be done by a day sail or a ferry ride. Both have their advantages and disadvantages. Our favorite is to get a private boat like Palm Tree Charters and go wherever you want with a knowledgeable Capt. Little pricey for two � great for six

Much like SXM, it is all best to pay attention to the cruise ship schedule to avoid crowds on certain beaches on certain days.

The drink of the USVI is a Bushwacker
The drink of the BVI (in case you do a day trip) is a Pain Killer

You will be driving on the Left Side of the Road. KEEP LEFT!
(Really not that hard)

If you like to hike � STJ has some amazing trails
If you like to shop � STT was made for you

Just a heads up for the Westin folks. A nice resort, but it is one of the few places on the island that actually has grass and is built on a manmade beach. Beautiful Pool.
To find the beauty of STJ a rent-a-car is a must!
For that matter STT you need a rent-a-car also.

I will let someone else jump in.