Both games are very difficult, but for different reasons. Ninja gaiden requires you to be extremely quick and you will die many times despite not knowing where or who hit you. Dark souls, in that manner, is more fair. You understand who hits you, and how they did it. Don't confuse that with it being simple though, Dark Souls will have many enemies that can do half of your health in a single hit, and some (mostly bosses) can wipe out your health in a single hit. In Dark souls, it is possible to simply die from not knowing what to do.
Ultimately, I believe that Ninja Gaiden would be more difficult on the master ninja level, but Dark Souls doesn't use the leveled difficulty (everyone has the same difficulty to start with), so I would say that if you were playing ninja gaiden on a lower level then Dark Souls might be more difficult for you. Not to mention that it can take up to around 50-70 hours to complete. I suggest trying it though, it is alot of fun and there is no harm in trying.
There are many games like halo. Try section 8: prejudice on the xbox live marketplace, or as someone else said, singularity. Crysis and the newer Crysis 2 are very halo like, complete with regenerating armour and a super suit with fast movement and high jumps.
As far as mature content, yes. There is one sex scene, as well as frequent cursing, and of course lots of violence.
As far as gameplay though, not really. Red dead is an excellent third person shooter. While the cover and run systems may not be perfect, and there is the occasional graphical glitch, the game does stand up to its good name. A great game for anyone interested in third person shooters to at least try.