Ninja Gaiden easily, Devil May Cry you can easily adapt to, you can get away with a lot of mistakes, Ninja Gaiden makes you pay dearly for even one slip up.
I've always thought that Ninja Gaiden had a difficulty curve equal to taking a single step and then having a ton of bricks dropped on your head.
Devil May Cry 3, while hard, was still a basic Hack N Slash, Ninja Gaiden requires a lot more strategy in your moves, planning and attack patterns and like I said, Ninja Gaiden can and will make you suffer for making mistakes.
So I vote Ninja Gaiden, I always will.