I will try to be as clear as I can. In the way of speaking there is not (much) a difference. As you will read on this homepage, the bujinkan exists of 9 traditional schools. The 9 Ryu. They are Ninja and Samurai schools. Only three of them are really Ninjutsu schools. The others are Samurai scools. The name of the orginazation is "The Bujinkan." The word that is mostly used when you are a member of the bujinkan is probably Ninjutsu. Ninjutsu is what you train and Ninpo is seen as the highest order of Ninjutsu. So when you use your skills, you learned from Ninjutsu, in real situations that you can call that Ninpo. I hope I explained this correctly otherwise send me an Email.