We've done all of the above. I mean we've done every kind of WDW transport from every resort, and also driven all over property. The answer depends on your priorities, as others have said. Always pros and cons both ways. Here are few factors:
Car is often the fastest, but not always. POR to DTD or POFQ, absolutely take the boat! From Epcot to the Epcot resorts, walk or take the boat. Specifically at POR, many buses loop around the bus, making many stops.
WDW boats stop during lightening and fog. Just threat of a storm is enough to stop boats. Sun might even be shining! Boats and monorail are fun and relaxing. If you want scenic experience WDW transport may be best. Monorail usually beats the boats, but the boat is more fun.
Cars = car seats for kids. Cars also = a designated driver. It isn't just adult beverages, you might just be tired at the end of a long WDW day. On the bus, you WILL need to lift and FOLD your stroller. Please don't be one of the evil people that endangers everyone by leaving it rolling into others and blocking the exit!
Parking at the lots includes the option of the parking tram, but walking is usually faster.
All of the above is also aimed at folks who are able bodied. If any member of your party has limited mobility - including but not limited to small children and seniors- then WDW transport can help maximize your party's endurance. If a
scooter or stroller is in your mix, then the monorail can't be beat. Buses with a scooter can be a hassle, but using your own car might not be an option at all if it doesn't fold.
If you drive- anywhere on property- know that are POLICE stops are VERY COMMON! SPEED LIMITS and SOBRIETY are ENFORCED! I find it also pays to use the GPS on our phone. Missing a turn can be a pain,and many WDW roads are one way.
Oh, while there's the monorail around the MK resorts, and shuttle boats link local hot spots (POFQ and POR for example), mostly WDW doesn't offer direct transport between hotels. If you plan to eat dinner at Boma then return to POR, you'll want to drive or pay for a taxi. Otherwise you have to do a two leg transfer that can be VERY slow.
We tend to a mix. We tend to avoid the buses. We walk and take the monorail whenever those are good options. We take the boat when we want to enjoy the ride/conserve our energy, but not when we are in a hurry. (Except at Universal, the US boats between the hotels are fabulous!)
Like I said, we've done everything many times.
