Will be visiting in a few weeks, 7 nights in St. John, and I was pondering the same question. Jeeps are ~$70/day on St. John this time of year. Have done enough research to learn another option is rent at STT and use the car ferry to STJ. So I booked a compact with Avis at STT. Should be noted no codes or coupons seem to work for STT, believe me I've tried!
But now I'm reading a 4-WD is best on St. John. If we skip remote places, do we need a 4-WD? Was told the same for our Vieques trip this year (plus they only rent jeeps there anyway) and discovered that a 4-WD was not necessary in Vieques (just skip remote beaches). Is St. John more hilly than Vieques?
For the price of an 8-day jeep rental plus car ferry (only 2 of us), that would pay for a lot of taxis. Other option (staying at Westin) is rent by the day when we want to venture out.