www vs non www, which is better for SEO?

There really isn't much SEO benefit either way as long as the appropriate redirects are in place.

My preference is the non www version as it makes a cleaner URL - which arguably (and measurably) has better performance in rankings due to the slightly increased CTR from the shorter and cleaner looking URL.

Much more important is having HTTPS instead of HTTP.

Pick one, stick with it and be consistent.

For SEO there is no real difference either way you prefer, Google have also backed this up in many of their videos. 

However there are a few things you need to be away of. 

• Don't mix on your site which way you link to internal pages.
• Accept both versions but redirect the one you don't want to the one you prefer. 
• Short URLS are often prefered so which is easier to read out over the phone for example.
• Always make sure your canonical tag points to your prefered domain.
• Where possible get any external links changed if they are pointing to the wrong version.

The one main thing to be careful of is redirect chains, so lets say you have HTTPS and don't want WWW. but someone goes to HTTP://WWW version and you redirect to the correct version. You should try and do this in 1 redirect. 

I see almost every site first redirecting the HTTP -> HTTPS then redirect the WWW to the non WWW. so 2 redirects before it gets to the page. 

As you control the structure of your site you should be able to go directly to the final page without having to keep redirecting users from 1 page to the next. 

One thing to note, if you specify the non www in Wordpress, the www version doesn't even work. A redirect is required.

The quick answer to the www vs non-www debate is that there is no significant difference between the two. However, if you dig a little deeper, you'll find that www domains have certain minor technical advantages that help a website operate better.