Our group has purchased diamonds, emeralds and the beautiful pink and dark rainbow topaz from Joe's in Phillipsburg over the course of two trips. We were satisfied. There is a shop sanctioned by DCL in one of these ports, cannot recall which, called Tanzanite International that had a huge selection. We looked in all the shops that DCL O.K.s Just keep comparing and you could get a good deal on either island. I prefer St Maarten, I just kinda like the place. A lot more funky than Charlotte Amalie. Go to the shopping talk and get a "preferred card" from the port CM. They'll treat you well.
Carla