This one is easy. If you are staying on Windows, then HDX is your choice as Carbon runs AVB and there still are no AVB drivers for Windows(sadly). As for i9 vs XEON, if you are comparing similar speeds, XEON probably has a slight edge(but an i9 is no slouch). If budget is tight, take a look at an off-lease HP Z820. Those can run dual 3.5GHz XEON 6-core. I think they max out at 128GB of RAM and are built like tanks(1100 watt PSU's
). Mine came with 12 cores(3.5GHz), 64GB RAM, nVidia Quadro graphics, DVD burner and a 500GB Samsung SSD and cost me $1100 shipped to my door. I added a USB3 card for more ports and a Behringer USB-SPDIF adapter(for all non-Pro Tools audio) and swapped the graphics card(so I could run 3 screens). Installed my HDN card and recorded full bands over the next 18 months with it. This machine is on Avid's approved list, so take a look at the setup instructions as HDN/HDX cards need to go into specific slots.
This also begs the question; do you NEED HDX? All my tracking was done at the 64 buffer and my usual tracking template had around 36 audio tracks, 8 stereo AUX tracks(with reverbs), 4 mono AUX tracks(talkback), a few VCA's and a MASTER track. Plenty of low-latency plugins and 6 sends feeding headphones. This was with HDN card fed by 2x192 IO's and 2xHD IO's). Of course, your needs may be different
