Yes, Ninja Gaiden Black is better than Sigma. But its not
vastly superior like some in this thread seem to be suggesting. Lets try to compare:
- Rachel missions. These are mediocre and should have been optional.
+ New music.
Its really catchy and fits the original game just fine.
- Doku's first form boss music was changed. The original was better.
+ New area in chapter 2 after the horse rider boss where you enter a burning building. Pretty neat.
+ New dual katanas. These are very well balanced and do not break the game, nor are they useless. The 360 attack in particular can deal good damage but is a fairly slow and deliberate move.
- You can heal with the D-Pad. Normally this would be a huge positive, considering having to go into a menu just to use a small potion is shitty. But the problem is you can heal during boss grabs which gives you greater margin for error which by extension means Sigma bosses are a little easier.
-/+ Removes some of the lame "puzzles", lever finding, and switch pressing. They add nothing to the game. Some people like it though (?)
+ New fiend challenge in the airship is nice, those enemy types needed a fiend challenge.
+ Greatly improves the water controls.
-/+ Adds new save statues and shops to certain places. I'm a fan of this but some people prefer the original placements.
+ Camera is slightly better in Sigma compared to Black. You can more easily use the right stick while you're moving with the left stick. Its not nearly as stiff.
-/+ The Berserker enemies are harder in Sigma. Using the Lunar's counter won't give you as many i-frames to combat them and even if they are off-screen they are far more aggressive in this version with using their energy ground attack. This can be seen as an imbalance.
- Random loading at a few doors and treasure chests.
+ The new enemies are all pretty neat. Motorcycle guards can be Guillotine Thrown off their bikes. Mermaids in the underwater level can be izuna dropped. The fat airship boss reskin you fight before the military base is nothing special, but I will gladly take it over the dumb helicopter boss (which you still fight in Sigma later anyway)
-/+ The Mission mode has been jumbled around from Black. Really weird arbitrary changes. One mission has you fight Doku, Alma, and Marbus at the same time in a small demon arena. Another has Rachel fight the Alma reskin and Ishtaros at the same time too. These can be crazy harder which is nice but they encourage spamming certain moves.
- The Sigma karma timers in some fights have been fudged with, for those who care about online leaderboards.
Sigma's are for those who can't handle the real games so opt for the kiddie experience and pretend they are still able to handle in the adult swimming pool...
I can't speak for Sigma 2 since I never played through it. Just from playing the Sigma 2 demo alone, I can say I wouldn't enjoy it very much compared to the original. But with regards to Sigma 1, you're wrong.
Black was the one that added the easy mode. And lets not pretend you can't spam izuna drop on humanoid enemies in ANY version of the game. Then charge up a UT with their essence, unleash it, and repeat ad nauseam. Ninja Gaiden is not an "adult" game, not even on Master Ninja. I conquered it before I was even able to legally have a job.