We always rent or bring our own car. Of course, we live here, so bringing the car makes sense, but my mom visits from Chicago so we use her rental. I couldn't imagine traveling any other way! We have APs, so that covers parking for us, we are able to drive to the resorts for dinners, we are able to get to stores if needed. Last time we visited, our entire family came down with the flu. Thanks to our car, we were able to drive to the clinic and get the diagnose and medicine that we needed. I find that the Magic Express, while great and keeps the tourists off our roads, can be limiting. You are on their schedule, must leave when they leave, I would feel so shackled!
If you do drive, remember the new Florida law about carseats - babies must have them! I would look into buying the GoGoKidz cart - worth it for the airport, trust me!