Hi Coalisgood,
Here some of my thoughts:
Dual Xeon 2687 v3 12 cores each = 24 in total
Max clock speed: 3.5 GHz
Price: 2 x US$ 2445 =
US$ 4890Price per core: US$ 203.75
Single i9 7900x 10 cores
Max clock speed: 4.3 GHz
Price:
US$ 970Price/core: US$ 97
(All prices from
www.newegg.com)
As you can see from the comparison above, the i9 is definitely better value for money. Moreover, it has a much higher clock speed (GOOD). It also supports up to 128 GB RAM. If you need more than 128 GB RAM, then you'll have to choose the XEON. But based on your posted job settings, you should be fine with 128 GB of RAM.
Regarding not enough CPUs and the effect on a machine with 4 GPUs. In the past this was a bigger issue, especially for the image alignment step (computation of tie points etc). But since version 1.3 PhotoScan uses not only CPU but also GPU power for this step, hence you should be fine.
You might actually be OK with just using 2 or 3 high end graphics cards (NVIDIA GTX 1080 Ti instead of standard GTX 1080) which will open up 1 or 2 more CPU cores.
To sum it up. The system I would choose/build (right now) looks like this:
-
Intel i9 7900X (water-cooling if possible)
-
128 GB of RAM -
3 x NVIDIA GTX 1080 Ti - all good things come in threes

Nice side-effect: It is much cheaper than a XEON setup. I would only go for XEON if I needed more than 128 GB RAM. If that's not the case, the extra cores/power are not really worth the
MUCH higher price. Also consider that XEON cores run lower clock speeds compared to their i7 or i9 counterparts.
I hope that helps.
Regards,
SAV