I think there are more children playing on xbox live then pounding away at a keyboard on MSN or AIM thats why this could be a more important issue then you perceive it to be.
If i had a 12-16 year old i would never let him play Xbox Live, im not an uptight bible pounding dude against swearing or anything but I just wouldnt.
I would however let him us MSN or the web, because when you enter a room on live you have no control over who else is there and what is coming out of their mouths.
when you are on the web you have to be looking for a porn site to find one, thats the difference to me.