I've found over and over and over that if you check orbitz.com or priceline.com for cheap rental cars it's often cheaper then the fare for two or three on an airport shuttle.
I've seen $20/day cheeepy rental cars from all the major rental agencies frequently on these discount sites! Worth looking into especially if there's more than 2 of you needing a shuttle (that could easily be over $70 on a shuttle)!
You do need to factor in a bit of gas, if your hotel charges for parking, and maybe if you can pick up your car in LAX
and drop it at Anaheim to avoid parking charges. Takes a minute to look into all this but could save some money!
I'm the biggest fan of Priceline ever! LOVE THEM! I've used them for hotels and rental cars all over the globe... they often put my airline discounts (as an employee) to shame!
The debate continues... Starwood or Hyatt... which is better