Which option is faster composite primary or AUTO_INCREMENT + composite UNIQUE INDEX?

Question

MySQL

Data (number of records):
product ~ 100K
product_rel ~ 3M
Queries:
INSERT, UPDATE - none
Usually executed, queries like:
SELECT p.*,pr.rating
FROM product_rel pr
JOIN product p ON(pr.product_id_2 = p.id)
WHERE pr.product_id_1 = 12345
ORDER BY pr.rating
Which option is better (faster) for queries (SELECT), and which type of database (MYISAM,INNODB) to choose
CREATE TABLE product_rel(
product_id_1 MEDIUMINT(9) UNSIGNED NOT NULL DEFAULT 0,
product_id_2 MEDIUMINT (9) UNSIGNED NOT NULL DEFAULT 0,
rating FLOAT(9, 9) UNSIGNED NOT NULL DEFAULT 0.000000000,
PRIMARY KEY (product_id_1, product_id_2),
INDEX IX_product_rel_product_id_2 (product_id_2)
)
or
CREATE TABLE product_rel(
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
product_id_1 MEDIUMINT(9) UNSIGNED NOT NULL DEFAULT 0,
product_id_2 MEDIUMINT(9) UNSIGNED NOT NULL DEFAULT 0,
rating FLOAT(9, 9) UNSIGNED NOT NULL DEFAULT 0.000000000,
PRIMARY KEY (id),
UNIQUE INDEX UK_product_rel (product_id_1, product_id_2),
INDEX IX_product_rel_product_id_2 (product_id_2)
)

Answer 1

The second option will hypothetically be slightly faster, but it is worth choosing for reasons of business logic - where will the table keys be used outside the database? Do what is more convenient, without thinking about speed. Or test the speed if you think that this will be a bottleneck in the system.
(Apparently, this is a table in the sense of something like "they also buy with this product ...", I would use the first option for this, without bothering with a separate ID.)