WWW or not WWW, what to choose as primary site name?
From technical perspective the only issue is traffic and incoming links (one of them should redirect to another). 

Now I need to choose which one should be primary. Some sites have www (google, microsoft, ruby-lang) and some without www (stackoverflow, github). Seems to me the newer do not use WWW.

What to choose?

Please with some explanations.
WWW or not WWW, what to choose as primary site name?
===

From technical perspective the only issue is traffic and incoming links (one of them should redirect to another). 

Now I need to choose which one should be primary. Some sites have www (google, microsoft, ruby-lang) and some without www (stackoverflow, github). Seems to me the newer do not use WWW.

What to choose?

Please with some explanations.