Hmm, I also think that it was the decision of the 4th. Maybe he wanted to get remembered as a true hero who saved the world from the Kyuubi. I also think that maybe the 3rd Hokage would have been able to seal Kyuubi. Or maybe he wasn't , it's nearly impossible to say how the things were. Of course it was easier to the 4th as he had the control of Gamabunta. It bet it would have been difficult to fight Kyuubi without Gamabunta.