Answer:
Using an external style sheet is always better for readability and maintainability.
External style sheets can be referenced and reused by many different pages. A local <style> tag could be used on each .html document, but this would mean that any CSS changes made would need to be updated within each style tag, on each .html document. By putting it onto an external style sheet, you only need to update the CSS in one location for it to be used throughout the website.