Have you looked into the cost of renting two cars vs renting a large van? Its probably a lot cheaper. Week long car rentals in Orlando are usually in the $150 range.
Fur us, we always prefer to rent a car. If you are going to be staying late for fireworks and parades, then I would prefer a car. The lines for the buses can be absolutely astronomical at the end of the day. But if you don't plan to do that, then it may not be so bad.
Also, do you have any early breakfatsts planned? If so, then realize you need to be walking out of your hotel room about an hour ahead of time to make sure you have plenty of time. And also if you need to get another hotel from your hotel, you will need to allow time to transfer on the buses.
All that being said, I have done the buses when DH was not around or when we needed to do something separately. I didn't think it was that bad and Sarah really enjoyed it. Its much more of a pain with a baby who needs a stroller and a fair amount of gear, but its easily doable when you have a lot of helping hands.
I think it boils down to are you the kind of person/family who hates waiting around vs the kind of person/family who hates driving in unfamiliar places on vacation.
And since you are staying at a WDW resort, you can use the Disney transportation from the airport and they will schlep carseats for you.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)