I've experienced El Capitan for a while and I've been very satisfied.
My new Mac has worked flawlessly, because El Capitan is a very stable and reliable system, a rock solid warhorse.
The hacker/pioneer in me, though, was tempted by the new features of macOS Sierra.
I'm very busy with my college, so I postponed this dilemma for a while.
One weekend, though, I created a clone of my configuration with Super Duper! and decided to give Sierra a go.
Boot time became much slower, several applications stopped working and I couldn't find anymore a custom sound I had created for one of my email rules.
I re-created it with iTunes, but I had to add it to system sounds for it to work.
As a result of that, this custom sound was played erratically every now and then, even when no email related to that rule was going through, driving me nuts.
A couple of applications stopped working at all, or stopped working correctly.
Scheduling a task with the at command stopped working. I could schedule the task but it wouldn't run.
I was very curious about Siri functionalities, but I believe there's still a lot to be done for it to be useful.
I understand maybe my individual accent might have given her a hard time, but I do not like the way it works.
Getting her to open my music and play a specific track, for example, was a little frustrating.
It returned the correct artist, but not the track I wanted.
After dealing with this misery for a week, I finally decided I was done with it and I wanted to go back to El Capitan.
I'm glad I created a clone before upgrading, because Sierra doesn't allow you to install an older version on top of itself.
The only downside to it was it took for me about six hours to restore my clone and go back to El Capitan. Other than that, everything works smoothly and I'm very glad.
So, if you ask me which one is better between Mac OS and OS X, I believe OS X is still the way to go, at least for a while.
Sierra is still too immature, as a product.
Maybe a fresh reinstall of Sierra could have solved some of the issues I experienced. However, looks like it has broken a compatibility path with applications such as SQL and, anyway, I don't have time for this.
I reserve to review these conclusions later on but, as of now, I don't recommend to upgrade.
Hopefully, coming Mac models will perform better with Sierra, but it still presents too many issues, even after update 1.