For 6-7 people your best bet is Biggger 45+. You will probably not be going into more than one marina during your BVI trip other than the one you start and leave from. Most of the time you will be on a mooring or anchor so the size will not be a major problem or cost factor as most mooring are $25 to $35 per night in BVI and if you go over to USVI the National Parks moorings are $15 per night.
The family with two 15 year olds you may get by with something smaller or chartering a REAL Boat rather than a Cat
I prefer a sailboat to a motorboat, and it is my belief that boat sailing is a finer, more difficult, and sturdier art than running a motor.... Jack London