We usually take the shuttle and then rent a vehicle for a few days to explore the coast. On our next trip we are renting from the airport for the whole time so we have more flexibility...lol mostly shopping flexibility for me
Some things that might factor into your decision:
What time of day do you arrive & depart?
How many in your party? Ages of kids?
How many days are you in town?
How many days are you planning at DL & DCL?
Are you thinking of doing other SoCal attractions (beach , Hollywood , other theme parks , etc. )
Is budget a concern?
Some of those answers will dictate that you will want a car.
Let us know and we'll give you an idea or two (hundred )...