Should I use include(_once) or require(_once) for loading php-file with customizer settings?

Should I use include, include_once, require or require_once to load my customizer.php file? That file has all customizer settings and code in it. What is your advice on this?

The file is currently being loaded with include() in the functions.php.

And can I use a fallback? For all situations that I can't predict, when the customizer.php file fails to load?