It depends on what features the client expects to manage.
FDM does not yet have 100% feature parity with FMC (and probably never will). Also, do they require historical logging? FDM will only give them real time logs - i.e. no way to see what happened yesterday (unless they are using a different external log solution).
Another option to consider is the cloud-based Cisco Defense Orchestrator (CDO). It recently added FTD support and is very nice. CDO doesn't do much more than FDM (although it does have event logging capability now) though since it uses the API direct to the devices just like FDM does.