session data depending on 'www.' in URL

The problem isn't with sessions but with cookies. The session id is stored inside a cookie. Cookie are by default linked to a single domain so they dont share data between domain.com and www.domain.com

Inside config.php you should set it like so (notice the dot before the domainname)
'cookie_domain'   = Set to .your-domain.com for site-wide cookies

This solved the problem. Great. Thank you.