This is probably covered somewhere, but I can't find it.
Matt Cuts talks about this often. WMT has a spot to set the preference.
Does is matter? Should I just pick one? Why should I care? What is standard?
tedster
7:31 am on Aug 29, 2010 (gmt 0)
Assuming you are not serving different content to the two differnt URLa, then what matters most is just picking one and sticking with it, including your internal linking.
You are better off to handle this canonical choice via 301 redirects on your server rather than in Webmaster Tools. And if you are already indexed one way predominantly, then choosing the other way can generate some serious but temporary drops in your traffic. temporary could go on for a month or two, however, depending on how tangled up things are.
Two other factors: 1. If you choose without-www then you've got shorter URLs by four characters - and that has advantages (twitter made this choice)
2. Even if you choose without-www, people will still type the "www" in automatically
