From experience, I'd say that if you aren't riding Disney buses, then rent a car.
I stayed at the Holiday Inn years ago and their buses were pathetic! School buses with no safety adptations for standees, always hot and late and dropping you off VERY far from the gate. They are on a schedule, so if you want to return to the hotel at 2:00 PM you may have to wait hours to get picked up.
Take time to ask your resort what type of transportation they provide. If the company is Laidlaw, you'll travel in school buses. Ask if the buses run regularly. They'll probably say yes, then ask how regularly they run. Is it every 30 minutes or every four hours? Careful here, they may say every hour but then you find out that they alternate. For example, the bus to MK comes by at 12:00 and at 1:00 it is the bus to MGM, at 2:00 to Epcot and at 3:00 to AK. So if you arrived at 12:15 and missed the bus to MK and it is where you wanted to go, you'll have a four hour wait. That is, assuming they're running on time.
Also ask: Are they air conditioned? How many stops at other resorts do they make before they get to the parks? How often do people travel packed like sardines in them? What safety adaptations do they provide for people traveling while standing (as you will unless yours is the first stop)?
There are preset parking spots at the parks where you are dropped off and picked up, but they are very far from the gates and never come as close to the entrance as Disney buses do. Have in mind that if your bus is packed at the park and yours is the first stop, you will have to struggle to stay near the door or squish your way our of there. It is hard on the kids.
If I did not stay onsite, I'd rent a car. But personally I hate playing musical chairs in and out of those trams (the lines in and out of MK can be neurotic), so I just stay onsite.