Difference between DirectQuery+Data Model in PBI and Tabular in SSAS

Hey guys, I don't know if I post my question to the right place, but this question now really confuses me.

We decided to build the Tabular in SSAS and use PowerBI just as our presentation tool. However, our client is asking for a flexibility of adding extra data source, which is usually an Excel sheet using Import by themselves in PBI. Obviously, if we expose a PBI report with connection to Tabular model won't give them this flexibility. In this case, I think the best workaround is to come up with a PBI report using DirectQuery and create the "Tabular" model inside the PBI report.

Now my question is, forget about my client's needs, in which situation, that DirectQuery+In report Data model should be replaced by SSAS Tabular model+PBI Connect Live?

Thanks!