1) Ansys claims that they started optimizing HFSS code for AMD cpus finally, and ver 2021 should run in par with Intel.
2) 3970x is ZEN2, and ZEN2 is weak in AVX instructions. Consider something from ZEN3
3)HFSS 2020 and newer badly scale with core number. More cores are better used for parallel threading in frequency sweeps and optimetrics. But optimetric parallel sweeping requires insane number of HPC licenses, which will not be used anywhere. Ansys says that HPC license number requirements for parallel optimetrics will be reduced in 2021R2. So wait for R2 release, or do not even try buying more than 8-core CPU. HFSS's solvers still love CPU clocks and fast multichannel RAM more than the core numbers.
4)Still, Xeon-W seems a better option, Just do not buy a retail machine from Dell - they have weak cpu power regulators and power-hungry Xeon-W CPUS are always staying in throttling mode. Consider DIY workstation with some advanced MB and cooling.
5) If you are not using anisotropic materials, particularly ferrites, you can seriously speed up computation with Quadro GPUs. Consider Quadro RTX 5000 or better. While these are very expensive (and use of Geforce boards is software-banned by ANSYS) they do not require many extra licenses; one GPU board works with a starters license pack.