In a vacuum:

* They would go the same speed. In a perfect vacuum, with no air resistance, both a heavy ball and a light ball would accelerate at the same rate due to gravity. This means they would reach the bottom of the hill at the same time.

In reality (with air resistance):

* The heavier ball would likely go faster. Air resistance is a factor that slows things down. The heavier ball, with more mass, would be less affected by air resistance. Think of a feather and a bowling ball. The bowling ball would be much less slowed by the air, so it would win the race down the hill.

It's about the ratio of mass to surface area:

* The real factor is the ratio of mass to surface area. A more aerodynamic shape, even if lighter, would cut through the air better and go faster. This is why a streamlined car can go faster than a boxy truck.

Let me know if you'd like to explore this with some equations or a specific example!