The Tobago Keys are great. Absolutely beautiful islands and beaches. No shops or anything, with the exception of a few locals selling wood carvings and fresh lobster.
Bequia is without a doubt my favorite island out of St Vincent and the Grenadines. The people are wonderful, there are great little restaurants, and secluded beaches. I have spent many days in Bequia, and might recommend staying a couple of days... or at least a day at the beginning and another day at the end of your trip.
Regardless of where you decide to go, I am sure you will have a great time. The Grenadines are a very special place!