wp-config.php sets the memory limit for the specific WordPress site which has that file. php.ini will effect your whole server. The php.ini is the parent setting, and so you can set WP to whatever you want within php.ini‘s range.
It’s also importan to note there is a WP_MAX_MEMORY_LIMIT where you can define a max amount of memory WP should use, especially if you have plugins gobbling up all the memory.