I guess it depends on how many people you plan on going to St. Lucia with. If there are more than 2 of you, taxis are not a bad idea. They aren't metered, but rather fixed prices. Splitting the bill more than 2 ways will definitely cut down on costs. However, if you do plan on doing various activities, then, yes, a rental car is the better way to go. Visit http://stlucia-guide.info/getting.around/rental.cars/ for a list of rental car/taxi companies and for general prices.