Why use www?
There are some minor technical reasons to use www, but those typically apply to the largest websites which receive millions (or more) of page views per day. These tend to be sites with a large number of services across several subdomains.
Why not use www?
Also known as naked domains, these make more sense today that we live in a world where mobile devices and small screens surround us. As a mobile user, typing an extra www on your mini-screen is a waste of time. Most mobile browsers tend to display only the naked domain you are visiting.
From a marketing perspective, it also gives more prominence to the URL, which may also be the companies' name or brand.
Do you have to choose?
Yes, you need to choose one and stick with it. The choice is yours, but make your website appear consistent to your users and to search engines by not having different versions. This includes always linking to the chosen domain and always sharing links (by email/social networks, etc.) to the same domain.
What do we recommend?
Naked domains. Non-www. Why? Because www. is merely a subdomain and nothing else. It doesn't add any value anymore.