Ninja Gaiden or Demon Souls

So, I just finished up Splinter cell , and am looking for a challenging game, I have only played the demo for Ninja gaiden but im wanting a fun, challenging game so naturally the souls series pops up.  

which do you guys reccomend more?

I've played both and would say Demon's Soul's for sure.   It's one of the few games I will remember in 10 years, thats how good it is.

Actually, the reason why I'm putting off buying a PS4 at launch (besides money, lol) is that Dark Souls 2 will be on PS3 in March, I think.   And when March comes, I know my PS4 would be collecting dust till Chrismas

They are two completely different games.  Ninja Gaiden is a hack and slash while Demon's Souls is an RPG.  You should definitely pick up Sigma 1 if you have not done so.  I am pretty sure you could get both titles for around $20. 

They are both great games, it just depends what you are in the mood for. If you want straight action, go Ninja Gaiden. Demon Souls is more of action-rpg.

Demon's Souls

Ninja Gaiden

Demon souls