I think you’ll be comparing x399 versus AM4 / x570, most likely:
From what I can tell the Threadripper will have all SATA HDDs connect via the chipset to the CPU. That means the chipset is the bottleneck with 4 PCIe 3.0 lanes in total for all drives. You’d have to calculate what bandwidth that would be. I’m too tired.
Interestingly it seems the x570 can have one or two SATA connections to the CPU, but it seems to depend on the motherboard manufacturer if they implement that. If they don’t the situation is the same as with Threadripper where all drives go through the x570 chipset. The difference is that the bandwidth is much higher with the x570 since it’s four lanes but PCIe 4.0, not 3.0.
In both cases it seems as if USB is available straight from the CPU.
Both have 16 lanes for your GPU straight to the CPU.
So anyway, I’m tired, but I’d be willing to bet that the 3950x is the better buy. My hunch is that the CPU will perform better as well as the platform as a whole. The only case where I can see the Threadripper be better is if you need the bandwidth of three m.2 NVME drives straight into the CPU.
Check Scanproaudio for DAWbench audio benchmarks, and Pugetsystems for video.