What is the difference between the two?
Noise Ninja is/was simply sw for noise reduction. Photo Ninja is a RAW converter/editor, which also does as one of its features, noise reduction.
I seem to recall a post somewhere that said the NR in PN was even a bit better than that of NN. I don't have personal experience with NN, so can't compare the two. But the NR in PN is very effective.