When should I use Magento multi-store vs separate Magento installs?

Use multi-store when your storefronts share most of the catalog, customer base, or admin operations, you get one upgrade, one extension stack, one hosting bill, and surgical per-store overrides where needed. Use separate installs when storefronts are run by different teams, have totally distinct catalogs, or need fundamentally different stacks (e.g. one Hyvä, one Luma headless). 90% of cases we see are multi-store wins, the maintenance and SEO consolidation alone usually justify it.