If you are planning on upgrading your site, you may be wondering if you should upgrade to Drupal 8, or if you should wait for Drupal 9. Why not both?
We have been promised that the upgrade between Drupal 8 and Drupal 9 will be simple. If you don't fall under these two specific use cases, why wait? Drupal 8 offers many improvements over Drupal 7 in many areas. Although the more modern architecture of Drupal 8 adds a little bit to the learning curve, the enhancements to caching, configuration management, and the templating system make it a worthwhile investment.
Here are my top 5 reasons to upgrade to Drupal 8 now:
Drupal 8 has configuration management baked into core. You can create entities, you can export those entities and can ultimately import those entities on another environment. Sure, Drupal 7 had features, but features is not as complete. Need to enable some modules, and disable others? Need to install that view you have been working on? Drupal 8's core configuration management can do all of those in a single call.
You could create an API in Drupal 7, but it took a little bit of work. In Drupal 8, RESTful web services are baked right in. With a default Drupal install, you are just a few clicks away from exposing a web service that third parties can integrate with.
Of this list, the introduction of TWIG in Drupal 8 is my personal favorite item. Drupal 7 used PHPTemplate for its templating engine. Although PHPTemplate is pretty simple and has served well, it is also wildly insecure. TWIG is fast, secure, and extendable -- it's a perfect fit for Drupal and has been largely welcomed by the community.
Contrib is Ready
Although it was a little bit slow, the Drupal contributed modules that you used for Drupal 7 are ready for use with Drupal 8. At this point, most modules have either been ported or their function has been forked into a different module.
Since upgrading to Drupal 9 from Drupal 8 will be simple, there really is not much to lose. Drupal 8 is secure and has a modern, extendable architecture. Since the Drupal team is dedicated to future upgradability, you can take advantage of Drupal 8 now, so you don't have to wait.