Google Analytics - best metric to measure traffic to a website?

Are both the www version and non www version accessible from the internet? This is not good practice. Decide on which one you want to keep and set up a redirect in your .htaccess file.

I notice you are not using a secure protocol (http:// and not https://). This will hurt you on Google. Look into adding an SSL cert to your hosting.

Then set up a Google Search Console account and tell google which you are using, what your location is, etc, etc.

Then go to your analytics admin and remove the version you are not using. If you have two tags on your site, remove the one you don't want.

You can use users or page views as the main metrics to understand how many visits you get. Sessions is how many different visits happened. These could be by the same person... Users is how many different users visited your site.

Both metrics are good to use depending what exactly you want to see.

Another good one is Behaviour > Site Content > Landing Pages. This shows which pages drove the most traffic. A landing page is the page where the user first entered and/or where the session first began.