Best way for rewrite url: - or ?

Best way to replace white space

[webstyler]

Best way to replace white space "-" or "_"

Page:
apache web server

can be

apache_web_server.html
OR
apache-web-server.html

What's better in your experience and why ?

Thanks

[g1smd]

Always avoid underscores and spaces in URLs.

The underscore is not seen as a word separator, and the underscore "disappears" in underlined links.

Spaces get turned into %20, making the URL%20very%20hard%20to%20read.

Use hyphens or dots or commas as word separators.