For User Experience
Non www, for sure. But regardless of your choice, redirect the other to the chosen one. A user comes to know that WWW could type least While such non-www not received an error page. Do not redirect could cause a bad user experience and sometimes SEO problens if search engine make some mistake.
For technical reasons
This answer depends because of way of cookies works. The answer still "non www" if:
- Is a small site. Will not have subdomains soon
- If you want serve your CSS, JS and images optimized for performance will use another domain and never your subdomain.
Remember, this choice can affect not only performance but also security in some specific cases, because a subdomain could steal cookies generated by the primary domain or just waste bits on each user request for every file.