WPMU functions over WP functions?

I am writing a plugin to create users if they don’t exist, and the following code does not work correctly. Specifically, if I send wpmu_create_user() an email already in use, it will not return a WP_error object, simply FALSE.

Now, if I use wp_create_user instead, I get the error. What would be the reasoning to use the WPMU function over the WP function?

if(!username_exists( $user )){
        $user_id = wpmu_create_user($user, $pw, $email);
        if ( is_wp_error( $user_id ) )
        {
            $message = $user_id->get_error_message();
            myLog("Error: " .$message);
        }
        if ($user_id == false){
            myLog("Error creating user $user");
        } else {
            myLog("User id " . $user_id . " created successfully");
        }
    }