Is the 9975WX faster than the 3975WX?

Yes. It has the same 32 cores and 64 threads but Zen 5 cores at a 4.0 GHz base and 5.4 GHz boost versus Zen 2 at 3.5 GHz and 4.2 GHz. Expect a large single-thread gain and a solid multi-thread gain.

Can I use the 3975WX in an sTR5 motherboard?

No. The 3975WX uses socket sWRX8 and the 9975WX uses sTR5. They need different motherboards and different memory, DDR4 versus DDR5 RDIMM.

Which is better for a new workstation build?

The 9975WX. It gives you PCIe 5.0, DDR5-6400 and a 2TB memory ceiling on a platform that will keep receiving support.

Does either chip have integrated graphics?

No. Both need a discrete GPU, so budget for one in either build.

Is the 3975WX worth it if it is much cheaper?

Only if the savings cover an sWRX8 board and DDR4 RDIMMs. If you are buying a new platform anyway, the 9975WX's newer I/O and memory usually justify the higher cost.

How much power do they draw?

The 9975WX is rated 350 W and the 3975WX 280 W. Plan cooling and PSU headroom around those figures plus the GPU.

AMD Ryzen Threadripper PRO 9975WX vs AMD Ryzen Threadripper PRO 3975WX

These are two generations of the same 32-core Threadripper PRO tier, so the choice is mostly about platform, not core count. The 9975WX is Zen 5 on sTR5 with PCIe 5.0, DDR5-6400 and a 2TB ceiling; the 3975WX is Zen 2 on sWRX8 with PCIe 4.0 and DDR4-3200. Buy the 9975WX for a new build that needs current memory and I/O; buy the 3975WX only if you already own an sWRX8 board and DDR4 RDIMMs. No current prices were provided for either part, so compare live store prices before deciding.

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