Steve,
like you, we have been to BVI's several times, and have also chartered with Horizon in St. Maarten and Antigua. Did ASA course/charter in the Grenadines. We loved St. Maarten! It is more challenging than the BVI, less crowded and we had awesome wind the whole time. Unlike BVI, you will mostly anchor, and need solid anchoring skills. We started in Simpson Lagoon, which is the only thing that could be considered a hassle... have to pay for use of lagoon, and wait for bridge openings, and clear out in order to visit other islands and the French side, but we had no trouble. Some might consider clearing in and out of different countries a hassle, but we actually wanted to experience it and learn how it is done... really quite painless. We headed up to Anguilla from Simpson Bay, fun sail and a beautiful place. Would stay longer next time. Had to clear in/out, they let you do it all at once. Then, to French St. Martin and Grand Case where we anchored two nights. Great restos, beach and shopping. Very French, which we loved. Then, on to St. Barts. Had a wild ride over to Anse de Colombier, one of the prettiest anchorages ever. Again, nothing was very crowded, and we were there in Feb 2012. Then, to Gustavia itself. Unfortunately, we were there on a Sunday... if cruise ships aren't around, the whole place is basically closed. We were underwhelmed. We were in outer harbor area, and it was quite rolly. Had a fast, fun downwind passage back to Simpson Bay, and anchored outside the bridge the last night, which worked well. Could do shopping/restos without entering the lagoon, which we did the last morning. In comparing the chartering areas you mention, you will find more challenging sailing/open ocean passages/more reefs and trickier approaches than in the BVI. Horizon is a great company and we seek them out now. Our charter this year is coming up, back to BVI, but with Horizon. We want to do Anegada, conditions permitting. I do get tired of the "musical chairs" phenomenon in BVI, with the mad dash for the daily mooring ball, but it is still a pleasant, beautiful place to sail. I think everyone should do Antigua at least once. Such a sailing mecca with rich history, challenging conditions, sailing over to Barbuda was truly amazing. English Harbour and Falmouth Harbor must-sees, and loved the Shirley Heights Jump-up. On the Grenadines, again, wonderful cruising grounds and incredible beaches/snorkeling and interesting culture. I do believe the Grenadines are for those with more experience, and would not suggest it for someone's first time charter. Also, the "boat boys" who come around to sell you stuff can be harassing at times, and this is something you have to get used to and take in stride. Everyone has a different take on things and different experiences, but hope this helps.
