ChipFlip
$199 separates the entry points of AMD's two Threadripper lines. The 9960X sits at $1,399.00 used, the PRO 7955WX at $1,200.00. Both are the cheapest way onto their respective platforms, which makes this a more practical comparison than the flagships above them.
Entry-level HEDT is still not cheap
Neither of these is a budget decision. By the time you add a TRX or WRX motherboard and the registered memory the PRO line wants, you're well past what a high-end consumer build costs. Going in knowing that prevents a half-finished machine.
The reason to be here at all is either core count that consumer sockets can't reach, or I/O that they can't provide.
Which constraint are you solving
Cores and clocks point at the 9960X. Newer architecture, better per-core throughput, and $199 more buys you a generation of improvement.
Lanes and memory channels point at the 7955WX. Eight channels versus four is a genuine bandwidth difference for memory-bound work, and the extra PCIe lanes matter the moment you attach more than a couple of devices.
The used-market wrinkle
Older PRO chips depreciate faster than their non-PRO counterparts, because the buyer pool is smaller and more specialised. That's why a 7955WX undercuts a newer 9960X here despite the platform advantages.
For a buyer that's an opportunity. For anyone holding a PRO chip planning to sell later, it's worth knowing the depreciation curve is steeper.
Bottom line
Take the 9960X unless you specifically need eight memory channels, ECC, or the PCIe lanes. Newer silicon, better single-thread, and $199 is a small premium at this total system cost. If you do need the platform, the 7955WX being the cheaper of the two is a genuine bonus rather than a compromise.