Thanks for all the info. I'm still a little puzzled, but it has boiled down to two things really.
The achivements and the graphics. These achivements seem to be something fun to fight for, but at the same time I want the most powerful machine capable of doing the best graphics.
I'll probably end up getting the PS3. I don't like paying for the extra content either, or to play online. Have I understood it correctly that I need to sign up for Xbox Live in order to play it online?