I agree with almost everything Jacob Gillespie says in his article about URI design for CSS Tricks. Especially about not including www in your url.

A URI must represent an object, uniquely and permanently

One of the most fundamental philosophies behind a URI is that it represents a data object on the Internet. The URI must be unique so that it is a one-to-one match – one URI per one data object.

I am not much of an SEO guy but from what I have learned from others it is not good to have users going to a site with the www and without the www. You want them all to be 301 redirected to one or the other. My choice would be without the www.

When you type in the url of a website do you really type in the www part of the website, do you even need to? I think it is safe to assume that most non web related people do not know you can get to a website without the www or the http part of a url. Even Google Chrome is going to be dropping the http part of the url. Now that is in just the address bar of their browser but still, why make the user have to type more then they need to.

The www. should be dropped from the website URI, as it is unnecessary typing and violates the rules of being as human-friendly as possible and not including unnecessary information in the URI.

The article is a great read, go check it out here.