Just ran a quick comparison build, attempting the +/- same specs for each XI, and Boxx Computers, and it looks as though you get more bang for your buck (or same bang for less) with XI.
Boxx Apexx 2 2403 - $3,413
Intel i7 7700-K Quad-Core 4.2 Ghz (OC to 4.8 Ghz)
32 GB DDR4-2400 (4 - 8 GB DIMMs)
NVIDIA Quadro P1000 4 GB GPU
240 GB SATA SSD (no brand) (will be replaced with Samsung 960 M.2 SSD)
** Shipping not included
Xi MTower PCIe Workstation - $3,040 (same bang for less... and a bit more?)
Intel i7 7700-K Quad-Core 4.2 Ghz (OC to 4.8 Ghz)
32 GB DDR4-3600 (4 - 8 GB DIMMs)
NVIDIA Quadro P1000 4 GB GPU
256 Samsung® 850 PRO SATA SSD (will be replaced with Samsung 960 M.2 SSD)
** Shipping not included
Xi MTower PCIe Workstation - $3,330 (more bang for buck... and for less?)
Intel i7 7700-K Quad-Core 4.2 Ghz (OC to 4.8 Ghz)
32 GB DDR4-3600 (4 - 8 GB DIMMs)
NVIDIA Quadro P2000 5 GB GPU
512 Samsung® 960 PRO M.2 SSD
** Shipping not included
Still not sure how comfortable I feel with being dependent on branded custom builds for our daily work, but Dell Precision and HP Z workstations are now both entirely dependent on slower Xeon CPUs with 4-12+ cores which are absolutely worthless using AutoCAD products... Until one-day, Autodesk decides to go multi-threaded... I'm not getting into that conversation here; just a statement of fact when developing specs for new hardware.
Cheers