How long is your journey? If you want freedom in between your journey then you should rent a car. If you want your journey a sleepy one then have a bus.

Why not train? Generally it’s faster and more comfortable than either car or bus.

I would not recommend bus for long journey’s unless it’s the only way you can afford the trip or you’re traveling in the relatively few areas where there isn’t great rail service.

Information needed for a recommendation:

Which countries?
Mostly cities or mostly small towns/rural areas?
How many of you are traveling together?

Well Adam, Oldlady is saying right. Train’s are generally faster than bus or car.