In Ubers, Metagross is better. Metagross's physical attack is 20 points higher, which is good for obvious reasons. Its STAB attacks are super effective against common threats like Xerneas, Marshadow, and fairy Arceus, while Kangaskhan's STAB attacks aren't super effective against anything. Metagross also learns ice punch, letting it cover the always common flying types and ground types.
In OU, both are banned, so both are useless.