With ADFS, you have to create the relying parties, and manage the ADFS infrastructure.
Using ADFS also means the partner you are collaborating with needs a similar SAML type infrastructure so you can federate.
Note that you can also federate your Azure Tenant with a partners Ident Provider as well
B2B on the other makes all this much simpler and easier to manage. You invite the guests to collaborate allowing for more self-service and allowing you - the resource owner - to focus on the application and not the authentication.
the B2B experience is only going to get better. Not sure you can say that about the ADFS experience, though I'm sure some will disagree :)