As a web developer, I have often been asked which frontend framework is better. My answer is always the same: both. Yes, they both have good and bad features, which sort of even the score.
Bootstrap is clearly getting more attention these days and the upcoming release of Bootstrap 4 will give its popularity a huge boost. It is also widely supported by many plugins.
The negatives aspect is its dependence on JQuery and JavaScript. The file size is also an issue.
W3. CSS does not have these limitations. It has more templates available and is very easy to learn as it is pure CSS.
Negative: With W3.CSS, there is almost an overlap with content HTML
Both are bloated though.
The best practice is to define your own CSS. My best jobs have been done by separating content from design and presentation.
As a developer, Bootstrap knowledge on your resume is not bad. In general practice, the use of either depends on the task I have at hand.
