Personally, I think Xbox Ninja Gaiden II on Master Ninja is far more difficult than Master Ninja mode in Ninja Gaiden: Black. Black is typically fair, in that if you're good at the game's core mechanics you can get through it conceivably with no damage. NGII often relies on luck, with framerate drops, atrocious camera issues, very strange Tests of Valor, some stupid AI (which can actually make it harder since spamming is more effective for you and enemies in NGII with the ramped-up number of attacks that provide i-frames), etc.
Of course, part of that may be based on the fact I never used healing items until Chapter 7. Still, I feel like MN on NGII requires both abuse of i-frames and prayer the game permits you to do them. The "unpredictable AI" in NG: Black is actually not that unpredictable, and if you're smart and fast most enemy attacks can be retaliated (blocked, dodged, intercepted) on reaction. In NGII, the camera, increased enemy speed/count, obsession with all attacks giving i-frames and the unholy obsession with explosives makes things much harder to react to.
Furthering my opinion, I feel than Sigma 1 is a great (no, amazing!) game, but only because of what it took (nearly everything) from NG: Black. Almost every change/addition made to/for Sigma from Black was for the worse. I could list 'em off, but to save time I'll just say there's no doubt in my mind Black is the best Ninja Gaiden game.
