Both of these CPUs have a boost frequency, which will come into play when using SketchUp.
(idea of boost is that 1 core can boost higher, but not all cores…so it’s perfect for SketchUp).
So the boost speed of a 3990x shoud be 4.3ghz while the 3970x boosts to 4.5ghz (so, in ghz terms it’s 4.6% diference…not really enough to make a major impact in SketchUp).
If you want to test Sketchup performance differences with CPU frequency then take any CPU you have and overclock it by 5% and see if you notice a difference on a heavy model (disclaimer, overclock at your own risk).
I have to ask though…do you really need that much CPU? I dont use VRay much, but many renderers now utilise the GPU much more than CPU…so I assume you already have at least two or three RTX3900s in your computer? Even in hybrid rendering mode (which is sometimes slower than pure GPU mode) you may find the CPU isnt used more than about 60% capacity. You’ll want to be sure about this if spending $30k on a new computer.
I have a rendering PC with two GPUs - this lets me continue to use sketchup while the renderer is using the other GPU; unlike CPUs, GPUs cant split the workload into cores effectively. Renderers can typicall use two or three GPUs within one compute, but you may want to check if that’s the case with Vray.
Also, most (good) renderers have a Render Farm type setup, so you can use multiple PCs for larger jobs. If you can connect these using a >1Gbit LAN cable, then the result is very fast. Most companies have lots of computers that can be used for “network rendering” - an app running in the background simply uses any spare CPU or GPU capacity and contributes it to the “master” PC…very good for animations.
Most proper render farms dont use 64-core CPUs, but they use more efficient ones and a lot of GPUs (the best CUDA or OpenCL ones, depending on the renderer).
If it’s sketchup performance that matters, then the fastest single core CPU (5950x or 11900k) plus a bunch of PCs set up as rendering nodes, would be a good strategy. You don’t always need a very expensive “all in one” solution because it can’t scale very easily and will therefore be obsolete earlier…(eg the 3990x is already last-gen).
On the other hand, it is quite handy to sit and do everything at one computer.