FortiCloud is geared towards smaller and simpler installations where there is only a need for more basic reporting. This fits many scenarios on the SMB end where they don' t want or need to purchase a fully featured FAZ unit.
FAZ has a huge subset of functionality - including the ability to write you own reports - that are not available on FortiCloud. You get IPS packet archive, DLP archive, much more storage for logs, log forwarding and aggregation, things like that
It depends on what your needs are. If what you need is basic logging and reporting then FortiCloud is a very easy " set it and forget it" service that will do the job.
If you need more advanced reporting, want archives, or want to store more than 200GB of log data, then you will want FAZ.
Hope that helps
Cheers!
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Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security