Depends really on how many people you have.
Car rental can sometimes be cheaper if you can get a great rate, but your hotel has to have free parking to make this worthwhile. I have had great luck with both Priceline and with Travelocity getting a car rental for a decent rate at LGB.
Taxi is more inexpensive than SuperShuttle if you have more than 3 people, because you pay SS per person.
Don't forget tips for both taxi and SS, those add to the overall cost, and you have to figure that you have the cost both ways.