I wrote into a magazine about this type of travel some time ago, there was a section on 4WD car vs. a motorcycle, and here´s a quick summary of that (...really it´s just my 0.02, and I admit, that I´m heavily biased towards bikes, but if I couldn´t do it that way, would have no problem taking a car then):

4WD Car, pluses:
+ Good comfort
+ Luggage space
+ Good shelter from the elements
+ Shelter in an the unlikely event of an accident
+ Parts possibly a bit cheaper, and definitely easier to carry spares
(+ Depending on the car, possibility to sleep, cook shower, etc)

Motorcycle, pluses:
+ You don´t have a metal cage around you, while you ride, so you experience everything, all sights and the scents, more ´pure´ or ´undiluted´
+ Usually cheaper to buy and equip
+ Usually better fuel economy
+ You don´t get stuck in traffic
+ Cheaper on ferries
+ Can go just about anywhere, where you can walk, even very narrow paths
+ In practice, much easier to get across rivers, that you cannot drive/ride through (can use any makeshift barge, etc)
+ In the cities, easy to park away from the street for the night (even into accommodation hallway, or even your room!)
+ MUCH cheaper to transport across the oceans, airfreight a realistic option (this one´s a really big plus)
+ Depending on model, more simple & easier to fix and maintain, than a car
(+ Motorcyclists are like a ´brotherhood´, so help or company is never far away (but I´m not saying, that travellers on a car would be left without help or company, either!))