Compounds and polishes serve different purposes. Compounds can be thought of as low grit sandpaper, removing deeper defects, while polishes are higher grit sandpaper, removing shallow defects and any left over hazing left from compounding. They have different types of abrasives in them to serves their purpose. What you'll need to use depends on the paints hardness, color, pad, polisher, polisher speed, and skill level. On hard paints, you can use just a compound and it's ready for waxing, but on soft paints you'll want to refine the paint after compounding (or just use the polish by itself) for the best possible finish.
