Outside of that (The main reason you buy a system to game on), while you have to pay for it, LIVE is going to be well organized, and an easy
to use interface. It makes keeping tabs on friends, and interests, as well as sorting your online purchases a snap.
There are a few titles on XBL you may prefer over the PSN, Wiiware, or Steam versions. I have heard some cases where XBL had the better version.
This can also be said for some of the retail releases. Netflix auto ques on XBL so if you watch a season of say, American Dad it will autorun all of the episodes
in a row.
If you're willing to invest in it, Kinect is proving more popular than the PS Move. I can say some of the dance, indie, and puzzle games on Kinect
are fun, and not possible on the other systems. The Gunstringer is probably one of the better ones imho.
Also the 360 can be used as a media slingbox if you have a Windows XP/Vista/7 machine, and the Live service is free on Microsoft's other platforms
This allows PC gamers to talk to 360 gamers, Mobile phone users to talk to the both of them, and so on. There aren't any real cross platform games outside
of Shadowrun, but it does show promise, as Microsoft seems commited to the idea of Windows 8 bridging all of their stuff together when it hits.