[Farhoof]

The 24 and 28 core have lower base clocks compared to the 16 core. But all 3 have the same 4.4 GHz turbo boost for pushing single core speeds (as reflected in the same single core geekbench scores). How this affects Protools real world performance is probably a different story but I highly doubt it's worth the extra 5000,-(!).
All I would want is faster audiosuite rendering, I can play large sessions (i'm in post) just fine on my old 5.1 cheesegrater since forever.

Regarding HDX, I would go either HDX2 or HD Native with voice packs. HDX1 is less powerful (for me) compared to Native since the entire card is filled with 'mixer' and I cant even run dsp plugins and there is extra delays due to switching native plugins to dsp mixer. You will need HDX2 though if you want 128 IO for Dolby Atmos.