Mastery Upgrades

Mastery Tokens can be spent on two upgrades (blue up-arrow at the top left corner of the barrels list). They are both unlocked with the Barrel Mastery. They both have the same costs.

More Magnets
Focus on this one first, unless you're a GS farmer. It's boost is massive.

More Golden Scrap
The second best currently. A must have if you plan on getting tons of Golden Scrap; if you don't plan on farming soon you should get the other ones first.