Don't bother making a day trip from St Martin because you won't have time to see the nicest bits of the BVI. Tortola has some nice beaches over on the north shore and I'm sure my friend Manpot would show you a good time for however long you had there, but, if you come to the BVI, you really need to get to the other islands as well. Whether you'll prefer it to St Martin/Sint Maarten depends on what you like there. I lived and worked in the BVI for a couple of years and had many vacations as well. I've also spent many months on the French side of St Martin and love Marigot and Grand Case, but, I avoid the Dutch side, nothing for me over there. Which you'll prefer only you can say but give the BVI a try, it's very different.