The Vegetation Paint and Vegetation Scatter tools are similar, but the primary difference between the two is that the Vegetation Paint tools allows you to insert vegetation using a brush, while the Vegetation Scatter tool applies vegetation across an entire selected surface.
Vegetation Paint offers you more control and precision when applying vegetation, as the vegetation will only be inserted inside the area of the brush, so you can adjust precisely where you want vegetation and where you would like vegetation not to appear.
Vegetation Scatter is used when you are not concerned with the exact arrangement of vegetation and you want to quickly apply vegetation across an entire surface or landscape. This is especially helpful when working with very large landscapes that don't require too much attention to detail.