Any guidance on when to use a Shared channel vs. a Guest account?

[Jason_Pruim]

Looking at when one may work better than the other.

[#2]

This is an excellent question. While I don't have much experience with Shared Channels yet, I have leveraged Guest Accounts for clients to satisfy security requirements. Specifically, with guest accounts, the inviting Microsoft Tenant can enforce that guests have to log in and authenticate their identity using multi-factor authentication. Further, the inviting Microsoft Tenant can even specify the MFA type (either passcode received via email or SMS-based passcode). This might be one factor to consider when determining between either option. I am curious to hear what others have experienced in this regard.

Update: One other major consideration is that with Guest accounts, the guest is exposed in other Microsoft 365 Apps such as SharePoint Online whereas with Shared Channels, the guest will only have isolated access to channels they are invited to. 

[#3]

I believe one of its big advantages is removing the friction that comes with guest collaboration (eg switching tenants, managing tenant notifications, etc). Shared Channels is a more elegant solution IMO, and sometimes that elegance can be the key to better adoption.

[#4]

One mega-important thing to remember is data governance.
As a guest user, each guest gets his/her own account in YOUR organization. At signup the gues accepts your rules, does your authentication, etc.
In a shared channel your org simply trusts the other guy's org but there is no formal OK from the individual user. The external user simply comes over to your place with his/her own credentials.
At no point in time, the external user has to acknoledge YOUR rules.
For users in countries like Germany this can have major implications. Not only regarding privacy but for example by the fact, that the user's data is stored someplace else than the user's tenant.
After all, the shared channel is NOT hosted on both tenants but just by the "inviting" side.