Lots of people have varied opinions. The problem is, people don't realize that demands don't just scale linearly for the sake of it. Demands scale with usage. The Operating System, to some degree, would definitely need more RAM if new features or services were provided. But the RAM usage doesn't necessarily scale with the Operating System requirements, but user software.
What's most important is that you have enough RAM, a powerful enough CPU and GPU and enough Storage to run your software. So first, ask yourself, what software do you have? What does that software require?
Satisfy that need first. Next, the potential for increased demands.
This is where most people will tell you to upgrade your available RAM.
More importantly, however, you should upgrade your storage. Is 128GB really enough for your needs? If this is your primary computer, I dare say not.
If you have money, still, to play with, then you should upgrade RAM if you think your software requirements may change. For example, if you intend to run Photoshop often.
Just saying...