In the world of Power BI, data transformation and enhancement are crucial steps in turning raw data into insightful reports. Two powerful features at our disposal are calculated columns and Power Query custom columns. While they may seem similar, understanding when to use each can significantly impact the performance and efficiency of your data models.

🔍 Calculated Columns: Ideal for row-level calculations that need to reference other tables or perform context-aware computations. They are evaluated after the data has been loaded into the model, which means they are stored in memory and can be used in further calculations, filters, and data grouping.

🔄 Power Query Custom Columns: Best suited for transformations that are applied before the data is loaded into the model. These columns are added during the data import process and are part of the query sent to the data source. 

Which one to choose?
Use calculated columns when:
↳ You need to perform calculations that span across different rows or tables.
↳ The calculation requires the context of the data model, such as time intelligence functions.
↳ You’re not concerned about the refresh time of the data model.

Opt for power query custom columns:
↳ The calculation can be performed at the source, reducing the amount of data imported.
↳ You want to minimize the memory footprint of your data model.
↳ The transformation logic is related to the values within the same row.

Remember, adding a Power Query custom column requires a full refresh of the table, whereas a calculated column does not. 
This distinction is crucial when dealing with large datasets or when seeking to optimize the refresh performance of your Power BI reports.

In summary, if your calculation is independent of the data model context and can be pushed back to the source, a power query custom column is the way to go. 
For context-dependent calculations or those involving multiple tables, a calculated column is your best bet.

Stay smart and choose wisely to ensure your Power BI reports are both powerful and efficient!