I think you have a lot more destination options sailing out of STT. - a few days in St John or the outlying USVI cays - at the beginning, end or split at both ends of your BVI sailing days -- the option of St Croix or the Spanish Islands to the West is another option easier from STT.
No ferny rides - easy and probably cheaper provisioning - you can be at your boat within an hour of deplaning - a lot more direct flights into STT can make your travel days shorter - all things to consider.
You will have BVI and USVI C&I to clear but as mentioned it is much different than the ferry experience - and there is that cost to figure on too - there is likely to be more time spent underway as the distances covered are longer - that doesn't appeal to all charterers <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" /> - many of the non-BVI anchorages are quieter and there are fewer bars and restaurants just a dinghy ride away.
Overall it worth a try if the circumstances fit your needs.

There are probably? a lot more private vessels filling the available dock space in STT - certainly there are less available spaces for new dockage although 4 have been proposed in the last few months they are years from completion even if permits are obtained. The BVI has not had the kind of cruise ship traffic that STT has concentrated a lot of its marine economic development on or the advantage that the offshore companies may enjoy in their BVI based locations and those factors influenced the growth of the BVI bareboat industry in decades past - also the many beach developments that cater to boaters have made the BVI an ideal cruising grounds for the changing demographics of charterers and multiple moorings have simplified anchoring and amplified the amount of vessels that can use an anchorage.

Led by pioneers like Charley and Ginny Cary in the 1970's the BVI promoted and brought bareboating to a major industry.