DS (nearly 12) and the 'gaming' dilemma.. Is Halo any 'better' than CoD? What about younger siblings?

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DH & I are not very 'pro' violent video/online games, so up to now the DSs have only had access to e.g. racing/ non-graphic fighting games on the PC/ Wii/ iPod

Since starting secondary however it's become evident to me just how big a part the discussion about games is in friendship groups etc. I brought DS1 & 4 of his friends back in the car and it was ALL they were talking about - who had which games, at what level etc.
It was very clear that DS is alienated by not having access to any of this stuff.

I am very and about having to 'bow to peer pressure' but DH & I talked about whether we should get him an Xbox (his preference over a PS3) for Xmas.
I'm just worried it will be a 'slippery slope' though. His best friend plays Halo (rated 16) all the time which DS claims is not as 'hardcore' as Call of Duty, but I'm still not sure.

Also I have no idea how we would prevent DS2 (9) from seeing/watching.
DS1 does not have TV/ computer in bedroom (and nor do we want to do this).

Would appreciate any views/ experiences/ advice.....

Mass Effect is a great game, but it does have romance (and a bit of sex in it) so it's up to you.

Could you rent games from somewhere like lovefilm, and you could try them out first? Then for example, you could look at things like Halo to see for yourself and make a decision? You've not wasted money that way, by buying the game for it to turn out to be inappropriate.
There are planty of games out there that will be suitable to play, like FiFA which i'm sure most of his friends will play too!

I have a ds this age and and as far as I am concerned he is just going to have to find something else to talk to his friends about