To me, a stay onsite is mainly worth it for the transportation. Although I now find better deals at nearby Marriott condos, and love the space of them, driving is the downside! I don't always want to drive on vacation! But I suck it up.
I agree with the PP - at least for the MK I would definitely take the bus. Parking there is a pain.
Going from your resort to another resort, I would drive.
Going to the parks, I would prefer the bus! Maybe a water park, I'd drive (so you can bring more things like dry clothes, towels, maybe snacks, etc...).
Sometimes we used to just go to the bus stop, and whatever bus came first, that was where we would go for the morning. It was a fun game.