More content does not mean better. The extra content in both Sigma games are shitty sections playing as Rachel, Momiji and Ayane. More often than not they kill the pacing and you want to get back to playing as Ryu asap. The giant boss battles in Sigma 2 also feel like they were rejected from a God of War game.
With regards to NG2 vs Sigma 2, the gore is a huge part of Ninja Gaiden II's identity and you can't just gloss over it with ethereal magic nonsense. It's like listening to a metal song with bleeped swearing, or playing Doom but every time you rip and tear the camera looks away. A completely pussified experience made by and for puritans. But there is not only less gore - many enemy encounters are completely stripped from Sigma 2. Look how quiet this Sigma 2 level is:
Compared to the original NG2:
And then on top of that, the encounters still in the game have reduced enemy counts. The legendary, framerate-killing* staircase battle (*which runs at 60fps on Series X now) is a shadow of its former self in Sigma 2. What did Team Ninja under Hayashi do to balance out the lack of enemies? Increased their health and reduced how often you delimb them. Ninja Gaiden 2 had unbelievable fights with 15 ninjas, a tornado of limbs and blood flying like something out of Kill Bill, and they still try to aggressively murder you even with legs and arms missing. In Sigma 2 we got protracted fights against just 6 or 7 at a time, less limbs flying and less amputees trying to jump you, and who take a boringly longer time to kill.
Sigma 2 is almost an art form in how to fuck up an exciting combat system.
As someone who never owned an xbox console and never played the original ng/ng2, going from Sigma 1 to Simga 2 felt like a progression in gameplay. I think it all has to do with perspective. Did i miss some of the gore from sigma 1, sure but it wasn't like it was completely eliminated. There was still stains, and marks on bodies, still decapitaions and delimbing as well as torso enemies crawling after you. The carnage still showed it just disappeared after 10 seconds or so.
My memories from Sigma 2 also stem from the vita release (as i double dipped on both games for that, and played that last). It added a lot of the gore back in. I also loved the added modes.
I thought the extra characters were great. Who doesn't love some sexy ninja, Ayane and bad ass dominatrix style Rachael , spear weilding Mumogji (well I guess the anti beauty people at "reeee" would hate them) , but i loved the variety and fan service. Plus outside of the game , the advert for "boob physics, was funny awesome.
Maybe if i played the other two first, or at all I would have a different opinion, but having never played them i loved both sigma games. Never played ng3 as i heard it was crap, Looking forward to playing that now.
I think people just getting into NG games will have a great time. I know i did back on the ps3 in 2008.
Also, From what I have read, the reason sigma 2 didn't have the blood spurts and purple mist instead , along with some enemy adjustment was due to the engine, not really censorship. Apparently it was based on Sigma 1 engine, instead of doing a port of ng2 engine to ps3. They upped the resolution and character textures but had to cut down and adjust some things.
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www.play-asia.com
plus i never tried the extra stuff from the Vita port 
