Neither,
In wordpress styles and scripts should be properly en-queued through your functions.php file rather than hard-coding them in your html.
The wordpress way of adding a new css file, be it bootstrap.css or any other is to use wp_enqueue_scripts hook.
The following code should be in your theme’s functions.php
/**
* Proper way to enqueue scripts and styles
*/
function wp_theme_name_scripts() {
// enqueue your style.css
wp_enqueue_style('style-name', get_stylesheet_uri() );
// en-queue external style
wp_enqueue_style( 'bootstrap', 'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.css' );
// enqueue your script
wp_enqueue_script( 'script-name', get_template_directory_uri() .'/js/theme_name.js', array(), '1.0.0', true );
// Other styles and scripts
}
add_action( 'wp_enqueue_scripts', 'wp_theme_name_scripts' );
This is a great tutorial on how to load css the right way in wordpress.
Check out WordPress documentation for more info on enqueueing styles
Update:
Underscores Framework already has the function defined in the functions.php.
Open your themes function.php and find the function named yourTheme_scripts(), and add the following line to include the Bootstrap library.
wp_enqueue_style('bootstrap', 'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.css' );
Your function in the underscores’s file should look like this:
/**
* Enqueue scripts and styles.
*/
function yourTheme_scripts() {
wp_enqueue_style( 'yourTheme-style', get_stylesheet_uri() );
//Enqueue bootstrap css library
wp_enqueue_style('bootstrap', 'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.css' );
wp_enqueue_script( 'yourTheme-navigation', get_template_directory_uri() . '/js/navigation.js', array(), '20151215', true );
wp_enqueue_script( 'yourTheme-skip-link-focus-fix', get_template_directory_uri() . '/js/skip-link-focus-fix.js', array(), '20151215', true );
if ( is_singular() && comments_open() && get_option( 'thread_comments' ) ) {
wp_enqueue_script( 'comment-reply' );
}
}
add_action( 'wp_enqueue_scripts', 'yourTheme_scripts' );