To me it also depends on how often you plan to use the car. When we went, we only rented a car for two days - we were more than happy to spend most of the time at the resort.
In addition, the Westin ferry allows as many round trips as you want at no extra cost. We ended up spending a day in St Thomas. Leaving from the Westin dock made it pretty easy.
One other thing I haven't seen mentioned here is where to rent in St. John. We rented right at the resort but others have recommended renting in Cruz Bay. The big advantage (really big) is that when you go to Cruz Bay, you can park for free at the rental agency. Parking is such a hassle that we always took the cab (and they also charge per person. To me, if I wanted a car for most of the stay, I'd rent in Cruz Bay rather than in St Thomas. It is a bit more expensive but there are upsides to it to.