In today's job market, is it better to become a WordPress developer or a (full-stack) Javascript web-developer?

Which can make a decent living faster and more reliably? (Decent living being like the median
american salary.) Which one has more job opportunities?

There is no single job market, so you would have to be specific about your location etc.

But broadly speaking, a full stack JS dev is more marketable generally speaking because they can do more.

That said, there is plenty of work for capable WordPress devs. Just scope out the opportunities local to you, or just follow your interests.