[Bilderback]

I cannot vouch for the Wii yet.
But we have owned an Xbox 360 and Halo3 since releases.

The Xbox 360 has a huge social networking capability that far surpasses other consoles
but children really have to be supervised due to its free-for-all nature.
(Halo 3 has alot of online potty mouths)
There are parental controls but the online voice communications are unfiltered.
(I hear that Microsoft is actually developing a filtered words voice software which is cool.)

The Xbox 360 has alot of nice features now with the latest NXE update.
You can watch streamed videos from Netflix.com (with a subscription),
share photos and files with friends and even connect directly to your PC.
Regarding content... it's Microsoft, which means there is alot of pay-per-content.
For instance, my daughter plays Phantasy Star which requires the game purchase itself,
the Xbox Live service and the Phantasy Star subscription($10 per month additional).
That does not include additional game related content like the expansion pack.
Same goes for Halo 3. You buy the Console, the game and the service just to be forced
to buy new map packs and additional downloads.
But you get my point.
You only need to buy the Xbox online service if you plan to play online multi-player.