Which is better for SEO?

Hello,
www or no www, which is better and why?

Why I can't see www on so many sites? What are the specific reasons that Google don't allow www??

 

IMO there is no role of www in SEO, many web servers are configured to show URLs without www and many are configured to force showing www in URL. for the google's part, I never ever heared that google dont allow www in URLs, in fact a URL with www looks more professional with www. that is another thing if you have a URL shortening script and you want to remove www from the URL to make it more shorter, you can easily enable or disable rewriting urls with or without www from the .htaccess file on your web server.