I have played Ninja Gaiden on the Xbox and Ninja Gaiden Sigma on the PS3. Although I haven't played through all the difficulties and didn't complete it either, I can confirm this:
1) Sigma has additional levels and weapons
2) Sigma is more forgiving.
3) I do not remember if the enemies are replaced by more difficult enemies, but I do know I found it easier to play through and as far as I can remember, you face less white ninja captains. In fact, playing Ninja Gaiden Sigma 2, I didn't even die once.
Not to say it's easy, because it's not, it's just more balanced.
If you are thinking of buying one, I would say in terms of content, you would be better off buying the Sigma version since they offer more, but if you prefere the insane difficulty then I would recommend you stick to the original xbox and 360 releases.







