Both are good in their own aspects so it depends on the parameters you consider to determine which one works best for you
If you’re coming from a Microsoft Excel background for example you would prefer moving to Power BI as environment as well as scripting resembles Excel quite closely. Also its rapidly evolving tool and every new releases is coming up with lots of new features. They also have good online forums/community available for help
Tableau is also rich in visualisation aspect. Usability is also well. But when it comes to pricing most cases it would be slightly above Power BI