
Well, I've done both modes of transportation and, being a veteran of many trips to WDW (8), I can tell you first hand that I would opt for the buses every time. The drawbacks to taking a car are these...
1) You must drive to the themepark and then park car. This may not seem like a big deal now, but it will be later when you depart the park. (If you stay for the fireworks plan on a long wait since there will thousands of you all trying to leave at the same time.)
2) You must take a shuttle from the parking lot to the transportation center. Now you will have a choice of either the monorail or the ferry to get to the entrance of the magic kingdom. If you go this route, opt for the ferry. It's faster and it's actually a very nice ride across the lake.
The monorail is also a nice way to go but it is usually very crowded with a long wait.
Now taking the bus from the resort involves the following:
1) Walking from your hotel room to the front of the resort to wait for bus. (I've never waited anylonger than 20 minutes for a bus and I've been there when the park was packed!)
2)Once you are on the bus, it will drop you off at the front of the magic kingdom where it's a short walk to the gate.
3) The return trip is also pretty straight forward.
Now I will say one thing.... If your mother does have trouble getting around, you may find the return trip (at the busy times) tough to get a seat (unless another rider is kind enough to let you have their seat). I've given up my seat many a time to allow an elderly person or young child to sit when you could see they were very tired. By the way.... I've seen many good people do this and I've seen a great deal pretend that you are not even there. In any case, depending on the time of day, if you leave the parks in the afternoon, you will have no trouble at all at getting seats. It's the evenings when you have to deal with full buses. Buses are still the best way to go to see all of the Disney parks. Hope this helps!
Have fun!
