If you have a domain and you want to refer to it in a single way, either with or without
www you can use IIS URL Rewrite to redirect requests.
IIS URL Rewrite is a module that supports configurations from your
web.config file to manipulate URLs. With it you can easily redirect from a non-www domain to a www domain.
You might think that
yourwebsite.com looks shorter and cleaner, and I would agree of course, however if you want to aim to a high performance website you will have to reconsider.