Parking at DTD is free -- it's kind of a Disney "mall." You will find that most people seem to like the on-property bus system just fine, altho sometimes there seem to be long waits in line. I think the bus may be better for MK, but driving is fine for all of them.
A lot depends on the size of your group, time of year, how long you'll be there, what else you might want to do, etc.
It can take quite a while to get back to your car after a long day at MK -- (after the fireworks, just getting to the front gate can be hard [crowded], then you have to take the monorail or ferry to TTC (the busses were picking up by the front gate, but I've read that that's changed to TTC, which would make the wait for the monorail or ferry even worse) -- then wait for the tram and ride it out to parking, then walk to where your car is parked (which can seem like a long way if you end up at the far end of the row as we often do). (This is the problem that I would most like WDW to fix -- it's not a problem at the other parks).
The last two times we've gone, we had two cars! And there were some days we used both of them for some reason or other. I like being able to get snacks, water, souvenirs, etc. at
walmart and grocery stores off-property and go to other attractions and shopping outlets. We do usually stay off-property, largely because we are in the minority of people who prefer having our own car to using the busses (and it's cheaper and less restrictive).
Unless you need a big, flashy car, rental rates are reasonable. Gas may actually be cheaper (or at least not more expensive) at WDW (unless you go to walmart).
Sorry I don't know specific driving times, but they're not bad at all.
If you took a vote -- you'd likely find that most people would recommend you use the WDW system -- there's plenty to keep you occupied without ever leaving the property. I like having a car, just in case, even when staying on property -- even if I just needed to go get cold medicine or pepto only once!
Whatever you do -- take patience and sense of humor, relax, and have fun!