Server will make use of the additional cores to process quicker. I'm not certain if the client does.
The Server will make more use of multi-cores to separate out different processes into threads across the processors.
For development with the FileMaker Pro application, it is not so useful in that rarely will you be generating more than one process at a time. This will even be more true if you are developing on this machine and your database is on another server machine.
More cores may be useful if you are running a number of apps simultaneously. Maybe you like to develop in FileMaker while your mail app is open and you're running a Netflix movie in a Safari window and Photoshop to work on FileMaker icons. But as important as more cores will be lots of RAM.
The question you are asking probably is looking at the Mac Mini that no longer offers 4 cores even though the duo core is now faster. My conclusion is that if you're just developing, the duo core may actually be faster, especially for single processes.