Use repartition because it always shuffles all the data, ensuring a more even distribution of data across partitions.

Use coalesce because it minimizes data shuffling by combining existing partitions, which is more efficient for reducing the number of partitions without a need for even data distribution.

Use repartition with a specific partition size to control the amount of data in each partition precisely.

Use coalesce to increase the number of partitions, which is useful when you need more parallel processing units.