Which one do you choose ? CSS is easy and available since a long time. A lot of people has worked for years using CSS. But Sass is fun and powerful.
I like playing with Sass. Especially because a lot of files in my work flow uses Sass.
I like Sass mainly because I had some help with that. CSS is standard and will be there for a long time.
It doesn't matter which one you use. In the end you will have to choose which is convenient for you.
CSS, SASS, LESS....the list is endless. Essentially, the concept of "LESS" is to "do more with less" - basically, you reference other CSS classes using @ to reduce the amount of effort in defining styles which already exist. Basically, you can do the same in standard HTML programming to call an existing style.
Front end development has become a lot more complex these days. Very hard to keep up. A new framework pops up every alternate week.