Personally I prefer non-www addresses except that I also always use the Ctrl+Enter in FireFox which automagically adds www. and .com to whatever I wrote into the URL bar. In addition there seems to be an actual issue when using non-www addresses which is that cookies created by your forum are applied to the entire domain. So if, in the future, you decide to provide some other content under say newcontent.example.com which doesn't require a cookie then the cookie will still be sent regardless and more importantly if your new subdomain does require its own cookie then things can become confusing fast?
So having said that, which do you use for your forum, www or not?
Also noticing that XF doesn't use www.