Pros of renting on STT: Cheaper cost and pick up from airport.
Cons: Some rental cos. do not permit you to take the car to St John.

Something else to consider:
Most, if not all, of the rental cos. on STJ are small, family-run businesses that are struggling to compete with the majors (e.g. Hertz, Avis, et al.). Indeed, I believe that is why the ferry cost is so high, to discourage bringing cars over.

Also, some STJ rental cos. that have locations in Cruz Bay will allow you to park at their facility while you are in town shopping, eating, etc. A major plus if you plan on hitting Cruz Bay.

If you are interested in renting from a STJ rental, inform them of your dilemma. After all, this is the off-season, so they will probably be willing to negotiate.

Good luck!

Additional Pro's to renting on STJ are if you have a problem the rental agency is local. Also, four wheeled vehicles are about all they rent in STJ. They come in handy on some of the roads. Also, if you are renting a villa not all roads/driveways are well paved or paved at all so four wheeled drive is a must.

Yeah, dont rent a CAR on STJ! Are you sure that the cheap faires on renting on STT are comparable vehicles??? Most of the rental places on stj have fairly new fourwheel drive jeeps and suvs and 360 is only a little over fifty bucks a day. Compare that to your room rates and your flight and I think you are being a little cheap to try and pinch the locals on STJ. Maybe you could stay on STT and just visit STJ for a day to save more money if that is your main concern. Then on STT, you can eat fast food and save more money....or go to Cost-U-Less, pack your rental up and bring all your food back to your room and eat in your room. If you want, you could sleep on the beaches.........and that is cheaper yet.

Sorry, I am just having a little fun with you and I hope you dont take offence.
I, also hope you take into consideration that everything on STJ will cost a little more.

I can only put in one more vote for renting on St John and not bringing a car over from St Thomas. You really will want to have 4WD on St John even if you don't go on the unpaved roads. I wouldn't want to have to back down some of the hills in my car as I've seen some have to do because the roads are wet and 2WD can't get my car up the hill.
If you have a problem (flat tire, lost keys, dead battery) on St John, you will have to either deal with it on your own or wait for help to come from St Thomas and the agencies in St Thomas will tell you point blank that it can take as long as 24 hours for them to get someone to you, especially on a weekend.
We've dealt with L&L on St John and can't say enough good things about them. Amazingly good service and the rates were within $5 or so of everyone elses.