I think that Phoebe Pike pretty much said summed it up. But since the question was asked directly of me, I will throw in my two cents.
I am a huge fan of gritty Spy stories. For example, I have watched all 7 seasons of 24, read every book that Vince Flynn has written, as well as most books by Brad Thor.
The Splinter Cell franchise has the same feel as the type of Spy Novels that I like. It strives for gritty realism. The gameplay requires you to be careful. There are no time limits. It's more about stealth than anything else. You must stay in the shadows. You can also create shadows by shooting out lights (which is great fun!). You always have a choice to use stealthy, silenced weapons or knock people out with hand-to-hand combat. If you go this silenced, stealthy route there are achievements - you are encourged to do so. I think that you can probably go thru most levels undetected "if you are good enough". You also have sniper rifiles and can use traditional "loud" weapons. But then more guards will come and your life will be much more difficult.
Metal Gear never gave me this feeling of gritty realism. I know that some people swear by the game, but it never just did it for me.
I hope that you like my answer. I guess that I'll have to wait for you to tell me when I get back in the office, because our Q&A system doesn't have a "thank" or "best answer" feature, or allow you to comment on people's answers. Someone should really implement a feature like that so HubPages can start to compete with leading Q&A sites...