I've had experience with all the games you said; CoD: MW2, Assassin's Creed, and Saw.
CoD: MW2 - I myself like the Call Of Duty series very much. I like a few first-person shooter games like MW2. I play it with my cousins a lot when I'm at their house most of the time during the holidays. We've recently got Modern Warfare 2 and it was VERY enjoyable. It's a better game than the past CoD games I've played. The only thing your really doing in the game is protecting the U.S from an invasion from Russia from an organization called Task Force 141. There's several big city like locations in the game, one of them being Washing D.C. I highly praise the game for it's multiplayer. It's just amazing. My other cousin, who's 13 right now, likes to play the game too. There's nothing in the game that's too violent or anything that will make your parents get upset over since my cousin's parents were the one's who bought it in the first place. I recommend you get this game.
Assassin's Creed: I currently own the game. I've played it for some time now, and I've realized it's not a big deal the way people say it is. The game takes place during the Third Crusade in the Holy Land in 1191. It's a generic Third Person Adventure game where you are basically going around different villages and towns, killing people along the way, in order to find the "Piece of Eden". You are searching for this item and murdering everyone who stands in your way. You are trying to find this artifact so that the main character, Altaïr, can regain his genetic memories of his ancestors. To me, it seems like a boring plot for being an Adventure title. And I also think there's way too much blood and gore in this game. It's a mediocre game to me, but I guess you would be able to enjoy it more if you were older. I don't recommend getting this game.
Saw: I saw the movie, it was okay. I got rented the full version of the game, I hated it. Unlike Assassin's Creed where there's a plot TRYING to be made a understood, this game has really no plot at all. It's a really BAD survival horror game. You just have to escape from Jigsaw Killer's Asylum hat's filled with traps while controlling the main character, David Tapp. You go around torturing people to give you information on where to go and how to get to certain places. There's one scene in the game where your supposed to cut open bodies and sift through their insides to retrieve a key. I got to that part and is was HORRENDOUSLY DISGUSTING. Gosh, there's TOO MUCH GORE AND BLOOD. Konami, the company who made the game, said the game was released in all expected regions WITH NO CENSORSHIP WHAT SO EVER! I really don't recommend you getting this game SGC. I returned the game I instantly thought it was horrible. The game should have been rated AO for Adults Only. It was a terrible rendition of the original movie, even though the movie wasn't even that good to begin with.
So, in conclusion SGC, I suggest you go with CoD: MW2. I love that game. It's a really good game and I suggest that you buy this game as your first M rated game. I am guaranteed you will like it as well. I rate CoD: MW2 a 10/10. Amazing.