Which is more egregious and sinful: marriage with the intention of mut'ah while believing in its validity, or masturbation while not believing in mut'ah marriage?

It is not permissible to marry with the intention of temporary marriage (mut'ah), nor to believe in its validity, because this marriage is forbidden and invalid. It is not permissible for a Muslim to choose between sins, because the obligation is to abandon all of them, based on the Almighty's saying: "And abandon the apparent sin and the hidden," and the Prophet's (peace be upon him) saying: "And if I forbid you from something, then abandon it."