I don't think there is a correct generic answer to your question.
But if using PSoS or WebDirect heavily you are almost certainly going to want more cores on your server. Even there, the nature of the operations being done can change what is optimal. The FMS script engine, for instance, seems to make good use of additional cores unless doing things like a Find, which tend to get stuck in the main FMS database engine process. Somewhat oddly, to my mind, FM SQL expressions seem to mostly exercise the associated script engine process.
Flip side is, if most of your CPU bound activity is locked into the core DB engine, then what you really want is that core to be running as fast as possible. This probably has the closest comparison to the FMP client situation, but must admit I've never looked too closely at how well FMP utilizes multiple cores. WebDirect can also hit the client's CPU fairly heavily too.
What's more, many database operations may be disk bound anyway, so for those it is really the disk speed that is your priority.
That probably only begins to touch all the nuances of this.
Yes, I concur with what all you say and is also my general knowledge of the situation.
One thing of note about the Mac Pro is its use of ePCI flash memory and that may have an important result on performance than the type of cpu. That is one real plus to a Mac Pro!
FileMaker Server can spread threads across multiple cores. But a connected user cannot use more than one core of the server at time (e.g., core bound). So a server with many cores will better serve a lot of FileMaker clients than any really fast individual core server. FileMaker Server is advanced in being able to take advantage of multiple cores for multiple users, but it is a really large programming step to go the next step where an individual process can be threaded across multiple cores and it really is past the target audience that FileMaker goes after for now.
The Mac Pro may be a better machine in that I want my machine to be a development machine with server on it (both FMS and FMPA), but the new really fast iMac 5K with i7 turbo probably would be faster on any individual process. But it sure would be fun to see a comparison with actual numbers doing a large cross section of functions.
I would assume that the Xeon is faster... but also... a major issue is the bus, memory and drives that feed the cores.
I always find that the Drive speed and bus speed tend to trump the CPU.
You should see some of the most amazing i7's.... doing nothing... waiting for a 7200 RPM drive... to do its work. LOL.
Your question is a good one. Frankly... I had this question in the past...and i had to write little FileMaker Template that would beat up on the client.... with a timer... and then chart the results.