Play Station 3. Overall the system is better and while you are paying 100$ more, its worth it. It has better online games (Free online) such as Warhawk.
Not to mention, the quality of the games that will be PS3 only that are coming out in the next year or so (Metal gear Solid 4, Gran Turisumo 5) is STUNNING. No X-box game has this kind of potential, and If the people at Sony know what they are doing (Which I know they do), over the course of the next year, the system will be utilizing itself for its full capacity.
Did you know that right now the average PS3 game is using only half of the systems power, because of limitations, but once people learn how to work with it for its full capabilities, the games will be amazing.
Not to mention the amount of user created content that is coming out, such as for people to make their own system themes which can be downloaded right from the internet (using your PS3) and installed on your system.
One last thing, don't get sucked into buying the 360 for Halo 3, because I have played it and their is NOTHING in the game that was not new or innovative.