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The xbox and the PS3 have a ton of games out there now and I'm sure you can find stuff your girls will like and you as well. I love the Halo series too and I used to play it quite a bit. I loved Little Big Planet which was avail only on PS3 and I loved Fable for the xbox. Both systems have their own games like that so it's hard to choose sometimes. Most people don't want to invest in multiple systems like myself and it does get kind of expensive.
If your the kind of person that is just looking for one system then you'll probably be a fan of that company for most of your life. I have friends that only play xbox and they swear by it. Then I got friends that only play PS3 and swear by that too. We get in arguements all the time about controllers, game features, etc. In the end both are quality systems with tons of games. I just lean towards PS3 as my current favorite because of the extras like Blue ray player for free and up until this year they were the only one with built in wifi.
Also xbox users have to pay to play games online. Not much like 50 bucks a year but PS3 owners don't have to pay anything extra.
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Just make sure you buy this game. Call of Duty Modern Warfare 2. This is the best shooting game I've ever played.