I'd go for an external CSS file if two conditions exist:
1) The CSS required is large in file size
2) The single page it formats gets updated frequently.
If you are constantly changing the content and not the design, it's beneficial to keep your CSS separate so that the markup and design don't have to be downloaded each time you update the page.
Otherwise, keep the CSS inside the HTML file. That's one less trip back to the server to retrieve a file. Sometimes the time saved by using an external CSS file is eaten up by the number of times a browser has to make requests to a Web server for additional files.