There are many factors that influences the choice. At the top of that list, whether we like it or not is time and money.
Second is where you plan to go which is again influenced by time/money. Most casual travellers do not actually have time to go off the main roads because they need to get from Cape to Cairo by a certain date.
If you have unlimited time, you’ll want to explore more off the beaten track. However, you will also not be too phased by any delays.
In my opinion, the third consideration is how you intend to travel. This is often influenced by money. If you plan to sleep in brick & mortar lodging all the way, you carry less with you which affects what bike you need.
Finally, are you traveling in a group, with or without support, pillion or no pillion?
There is no one perfect bike. Most often it’s the bike you own simply because you already have it and are probably comfortable riding it. It’s also a very personal and subjective concept.
In the end, what is more important is that adventure spirit as Michnus said, a healthy dose of it and the desire to travel. When you have that, almost any bike will do because it’s about the journey, not the destination.