The xbox 360 system uses a 'dedicated' online service that you subscribe to. It costs £40 a year. For this you get dedicated servers with a unified online service. You have one freinds list that is unified with every game, you get a little popup when one of your freinds comes online irrespective of what you are doing, whether it be watching a DVD or playing any game.
All the games for the 360 that have online capability use this same service making them all linked, or 'unified'. For example, you could be playing PGR3 blatting around the nurburgring racing a group of french noobs, when one of your freinds, who's playing Gears of War chainsawing people in the face, can private chat you by simply pressing the center button on the game contoller. the game does not get interuppted and you can chat away to each other. If he's getting owned, you can choose to jump to his game to help him out. It ejects the disc for you and you put in Gears of War, it then loads straight into his game without any prompting. All the time, your still chatting to him without any break in communication. You only have to have one list of freinds instead of the PS3's individual freinds list for each game.
Then theres the downloadable content. The 360's list of available is astounding with new demos being released on it every week, along with movie trailers, music, game trailers, full games (live arcade) etc etc, the sheer amount of content available has the second highest content rating in the world, the first being I-tunes. So you never need to by a magazine again as the demos, trailers and movie clips are all part of the service.
All the movie content is High Definition stuff, so looks awesome if you have a screen capable of displaying it.
Because the servers are dedicated, it downloads the stuff as fast as your connection is capable of.
It also has a queing system, on the PS3, when you choose to download something, you cannot que anything else up, your stuck waiting for whatever your downloading to finish before you can do anything else. The 360 can que 8 different items, while waiting for them to download, you can play any game you want and it won't make any difference, it'll still download in the background. You can also turn off the machine or lose you internet connection, it'll remember where it got too and resume the next time it turns on. Then a little popup will tell you when its finished.
Then theres the fact that it is a media center PC at heart, meaning if you have a wireless network at home, you can go into media player 11 on your PC, select everything you want to share in the network, it'll come up saying its found the 360. Then, go downstairs, load up the 360, and it'll link to your PC wirelessly, you can stream music or videos straight to the 360 via your home network. So you can watch all that porn you've downloaded onto your PC on your huge TV downstairs through your 360.
You can use your music saved on your PC to stream at anytime to your 360, meaning you can be playing any game, and have any music you want as a custom soundtrack playing in the background.
With the PS3, future online games that demand high bandwidth usage may require you to pay a monthly payment to play the game online. EG - world of warcraft. This is currently under disscussion between Sony and game devs. you could be looking at giving your bank details to different game devs everytime you get a new game and go online with it.