The basic difference in the diving on St. Croix and Tortola (and the rest of the BVI and even St. Thomas and St. John in the USVI), is the way the islands were all created. The BVI and rest of the USVI is volcanic in nature and thus has flat, continental shelf type of diving. St. Croix was formed tectonically (the merging of the North American plate under the Caribbean plate) and thus sits atop a wall......a 13,200' wall that is almost vertical in places. That's the main thing that sets us apart.
In addition St. Croix also has reefs (the largest living reef of any Caribbean island and the second longest barrier reef in the Caribbean), wrecks (5 within 100 yards of each other), and a Pier (the famous Frederiksted Pier which, in my opinion is one of the top 10 dive sites on the planet, especially at night).
There's the differences. You'll love the diving regardless of whether you choose St. Croix or Tortola, we just feel the diving on St. Croix is much better due to the reasons above.