Closer than most comparisons
Omni and Sigma are genuinely close substitutes. Both connect to warehouses such as Snowflake and BigQuery and query them live, and both offer spreadsheet-like interaction so finance and operations staff do not have to learn SQL. That overlap is why they show up on the same shortlist. The difference is philosophical: Sigma makes the spreadsheet the primary interface and governs through workbook standards, while Omni puts a semantic model underneath and lets the spreadsheet-style workbook sit on top.