I have been to both Scrub Island (this past March) and the 2 previous years to the Ritz on St. Thomas. My wife and I enjoyed both hotels very much. Scrub Island you are basically there unless you take an excursion to another island. We stayed 5 nights and didn't leave the island while we were there. It was very relaxing and just what we needed coming through a tough winter and a whole lot of long hours at work!!!
With St. Thomas I believe you have more options to see other things without leaving the island and can also take the ferry over to St. Croix for the day. The Ritz is the Ritz and the staff is very friendly and helpful.
It's just a matter of what you are looking for.....
Scrub Island is incredibly relaxing and quiet. On both of our previous trips we were so relaxed that we'd find ourselves on the balcony after dinner with some Wine and usually fell asleep for a few hours. Can't even tell you if the TV's in ther rooms worked as we never turned them on. The photos do not lie on the resort sit as it actually is that lovely.
Both times we've stayed there we enjoyed a 1BR King Villa with a full kitchen so we were able to cook our own meals most days. We took the Scrub Island ferry to Tortola where you catch it from the airport to Scrub and found a couple of stores so we'd could stock up on things for breakfast, lunch and dinner. The resort also has a nice Deli that is fairly well stocked if you decided to go that route on Scrub Island.
Here is some threads from both of our trips to the resort as well as another thread that some Insiders have asked. Please, if you have more questions ask!!
Hope this helps even though I cannot comment on the Ritz on St. Thomas.