When using a rental car on St. John, I know of two options: 1) rent on St. John or 2) rent on St. Thomas and take the car ferry over to St. John. Can anyone expand on these options? Maybe offer some advantages / disadvantages. Im deciding which I should do. Thanks!

If you're staying on St. John, then rent the car on St. John. If you're staying on St. Thomas and renting a car for the week, then take it over on the ferry. If you're staying on St. Thomas and not renting a car on St. Thomas, then rent a car on St. John when you visit. My two cents.

most st. thomas rental companies will not allow you to take your car over to st. john. a couple will; i don't recall which ones.
also i feel a car is a better choice for the roads on st. thomas and a jeep more useful on st. john.

We have done it both ways when staying on St. John. If you rent on St. Thomas, you can head right to the car ferry without the hassle of moving your luggage from one place to the next. I enjoy the car ferry and the ease of doing it that way. We have rented from Dependable Car Rental and Budget St. Thomas. Suffice it to say that we will never rent from Budget again.

The only disadvantage that I see is if you were to have mechanical issues with the vehicle while on St. John. They will not come over to provide help and you must find a way to get the car back to St. Thomas. Also, several car rental places in St. John offer parking in their lots in Cruz Bay. This is convenient, especially during busy times of the year. With a St. Thomas rental, you're on your own.

Renting on ST. John has been no problem for us and we are doing so next time. I just prefer to do so on St. Thomas.

Hertz is the only St. Thomas car rental that forbids you from taking a car to St. John. The others allow it but you may pay a surcharge if you break down & need a repair while on St. John. Just ask the rental agency but don't attempt to do it with Hertz.