I had never heard of LSIF. Its webpage says it intends to "define a standard format for language servers or other programming tools to dump their knowledge about a workspace". SARIF being targeted towards static analysis tools results, it seems more straightforward to support it both for producing results and for reading them.
To be honest, since both formats are at least partially developed by Microsoft, they might have some common people behind them, but as a user and producer of static analysis tools, I'd prefer to have a single standard. SARIF is already standardized and gaining traction, so it seems the better choice for now.