Physical
Both systems are large portable laptops.
The X500 (Launched June 2011) is 16.1" x 12.6" x 4.6" (933.2 cubic inches) and the NotePAC-III-PRO-V (Launched February 2019) is 16.75″ x 12″ x 4.5″ (904.5 cubic inches).
The NotePAC of course does feature a triple display monitor, so it’s not quite apples to apples.
The NotePAC-PRO has two PCIe expansion slots as standard, whereas the X500 is expandable using a ‘slice’ that replaces the bottom cover (the dimensions listed include the slice). The X500 slice can be PCI or PCIe, so if you need to support a legacy PCI card then the X500 is the one to choose.
Typical configurations of the NotePAC are considerably more powerful than the X500 and include an Nvidia Quadro RTX4000 graphics card in one of the PCIe slots.