Ideally, you wan to combine both methods. Using a framework is a great idea because when the framework puts out an update, it easy to keep your solution updated and then just work the kinks out. Also it saves you from having to re-invent the wheel. So much is created for you already.
Using a SASS or .LESS allows you to do many techniques that will make your code more organized but most importantly you will create way less classes because of the ability to nest. For example, in a normal css, you would create several classes to make each element behave a certain way, and if you are good, you'll do some mapping. In .less you would simply nest your code from div to span and the compiler will build it for you.
The ability to create mixins and imports make your working platform smaller as well. With imports, you could have 6 css in your your root, but you will be able to declare just a couple of stylesheet because they are all imported. Thus makng your page loading time much faster.
For instance, I am the senior developer for the Pennsylvania Department of Welfare and I run an entire state-wide website with only two css declarations in the head section. 1.The bootstrap.css and the custom.css that I created with .less to modify the bootstrap as I needed.
Hope that clear things out.
SASS