Azure MSSP subscription vs Tenant

I would probably opt to do separate tenants as it provides a higher level of isolation with separate identities rather than a single shared Entra ID space, granted it does add more complexity than managing separate subscriptions within the same tenant. So ultimately deciding between the two models depends on the level of isolation you desire between customers and the complexity of management you're willing to take on, as well as any regulatory or compliance requirements that might apply to your situation.