Well, as it has been stated before, PC has better graphics, but the 360 is cheaper. I find there is a better experience with the 360. When i play on my pc (a shooter) it just doesnt really fell like i am engaged in the game, where as with the 360 i do. I would say get the 360 and just use the pc for work, internet etc..
As for online play you get Xbox Live Silver for free, allowing you to download MOST demo's, video's, and MOST game add-ons.
Xbox Live Gold allows you to access all of the above, but allows you to play online. Gold for one year costs $90 Australian dollars, about 45 British Pounds.
YOU CAN ONLY PLAY ONLINE IF YOU HAVE GOLD!
I tried using the momo racing wheel on my 360 and it doesn't work.. Thats how i know.