What is happening with that world? For the first time I see someone says "vanilla" CSS.
And again why do you limit your question to only 2 options. Do you know there is also PostCSS?
You won't believe me: I am writing just a CSS, just in simple .css files and I just include them in HTML file, however, I am using PostCSS to combine all imports into single file because of perfomance reasons and I am using CSSNext which is like Babel for JS. I am writing CSS4 vars, "mixins" and other stuff which is transformed back to CSS2/3. CSSNext is a bundle of many plugins so you don't need to use autoprefixer, for example. It is already there.
But again, this IS plain CSS and my code works in Chrome/Firefox without CSSNext, same as my JS. Only when I need older browser support I am using simple tools to transform code and I never use complicated build configs, gulps, grunts, browserifies, whatever else.