They are defined as follows:
define( 'ABSPATH', dirname(dirname(__FILE__)) . "https://wordpress.stackexchange.com/" );
define( 'WPINC', 'wp-includes' );
dirname
is a PHP function that returns the path of the parent directory, and wp-includes
is pretty self explanatory.
I would say ABSPATH
is better because it’s one of the first things WP loads and it also looks better:) But there is no real “right way” because they both work.