Using relationships vs merge join difference

When you load the data to Power BI, only a single table (MergeQueries) that combines the two tables is loaded to the data model. The original tables won’t be loaded into the data model as you will be disabling the load, hence less memory consumption and better performance.

When you compare the two PBIX files with the same imported data, you will see that the Relationship file is larger in size than the Merge file.

However, its depends on the scenario why you want to use Relationship or Merge in Power BI, like using some DAX functions for analysis that may not work when using Merge. Also, if you have a complex model, it is recommended to do as many modelling at the Datasource level. The less complex the model is, the better the performance in Power BI.

Refer the below link for deatailed explanation 

MERGE VS RELATIONSHIP IN POWER BI - Ruki's Blog (rukiatu.com)