The simplest answer is:
- If you're only looking to collaborate using Teams shared channels (reference is below) then you may get away with B2B Direct, which is a basic offering targeting this one very specific form of collaboration. So, this is a very simple offering suited to very basic customer needs;
- If you're looking for broader, enterprise-grade integration where external users from other tenants can access the many of your organisation's applications and services, then you want to choose B2B Collaborate. Suffice to say then that this is the more complex-yet-robust offering that satisfies diverse customer needs.
They're two very different offerings with huge differences in scope, and because of these significant differences, there's not much benefit in looking at the technical details of either model as choosing one will almost certainly be because the other isn't appropriate, making the technicalities irrelevant.