Since the first upgrade from the original Urchin collection snippet, we have always recommended upgrading as each new iteration is released; the result of which has always been more features and better tracking; however, this is not necessarily the case with gtag.js.
- The ability to more easily implement advanced tracking with the use of custom tasks.
- Third party plugins like AutoTrack (developed by Google Engineers to easily expand past simple pageview tracking) are also not supported.
- GTAG is not available as a template in Google Tag Manager.
What, in our opinion, is the purpose of GTAG.js? The answer may be in the snippet itself: a smooth path to upgrading to Google Tag Manager.
You’ll notice that GTAG actually invokes Google Tag Manager (GTM) in the snippet. The Global Tag is actually a “GTM light” that has the snippet of GTM and the syntax uses the data layer of GTM to hold values used in the Global Tag. That said, the most important feature of GTM that the Global Tag doesn’t have is the GTM user interface to easily expand your tracking and layer in robust, advanced features.
If you implement GTAG, you essentially get a stripped down version of GTM that automatically handles your page level GA tracking and can pass data back to AdWords but nothing else.
All of this leads to this question: Should you make a change? Our recommendation is to deploy analytics.js via Google Tag Manager. Why? Safe, easily tested, nimble implementation of Google Analytics (and AdWords conversion tracking) that is easy to update over time.