site with prefix of www or no prefix at all?
what if the site has the www prefix before then turn into no prefix? why it is happening? is it because they change to a hosting company or what? they gain PR rank before but lost the PR because the URL site change into no prefix.
Thanks in advance 
You can choose either of them but not both. If your domain has both you will see mixed results because Search engines consider these two as two different links. 301 redirect to canonical single version is good, and cant hurt your ranking.
I asked a related question today. As long as you pick one, you are fine. Personally I like the www. although I have no idea why.
If you do a google search for .htaccess and www you will find a quick way to make it happen.
This is exactly right. Another option is to use rel=“canonical.” The heart of this issue is duplicate content. Here is Google’s two cents: