Price out all the options, and decide the value of the car during certain parts of your trip. Price specific rental companies out of their specific locations, I mean get really exact, and see.
On the Transportation board there is generally an ongoing thread about rates for rental cars and coupons/codes. Those work for just about anywhere. You can also check mousesavers for coupons and codes.
We had an 8 day stay without anything planned other than
Disneyland (did end up going to San Diego for a day to visit my brother, which was absolutely lovely), with a parking fee every single day, and it was still worth it. We went to two WeightWatchers meetings at the proper center (not just a hotel meeting), which are VITAL to our goals, we went to Trader Joes three times for fruit and water and other things, went to the Target over near both of those places, a Chase branch (forgot to pay rent before we left), and even to
Best Buy. It was extremely worth it for that flexibility and ability to do *whatever we wanted to do*. And we got a darned good deal. AND my husband is Emerald Executive with National, so we got a very spiffy car for a midsized price.
If you do *any* grocery shopping, or would *like* to get groceries; if you think you might want to go to an offsite restaurant (we went to Souplantation, yum! it's related to Sweet Tomatoes, where we always go at WDW), etc etc, then price it out and see if the value is there for the full vacation.