The real question: model first or spreadsheet first
Power BI and Sigma land on the same shortlist when a company wants to graduate its Excel power users into proper BI, and they take opposite routes. Power BI asks a modeler to build a dataset first, using Power Query to shape the data and DAX to define measures, and then lets everyone else slice that model in reports or from Excel. Sigma removes the separate model step for the person doing the analysis: a workbook looks like a spreadsheet, each formula runs as a query against the warehouse, and nothing is copied out of the source. Which route fits depends on where your data lives and who you trust to define metrics.