Taxi is around $26.00 each way, rental car is $40-50 with another $20.00 for license. Pay in US dollars - not Eastern Caribbean. Take cash for ferry and gas and departure tax.
Depends on what you want to do, I personally never recommend a rental on the first trip as I think people should sit back and relax and take in the sights without having to worry about directions or driving on the left. If you do rent a car and go to Shoal Bay East, do not get conned into parking - park at Elodias, buy lunch and rent some chairs. Parking is free on most beaches.