Greetings!
Very short answer: considering your customer is going to install two Aruba 8400X at the Core of its network...the natural step would be for sure to deploy them using VSX (just think about a VSX feature like "Active-active layer 3 default gateway"...not to speak about VSX LAGs and all the new features available on ArubaOS-CX 10.03, not counting what probably will land with 10.04 during 2020).
Maybe if ArubaOS-CX Virtual Switching eXtension (VSX) Guide for 10.03 wasn't clear enough a latest VSX Deep Dive presentation would blow most of your doubts away defintely (the latest I'm aware of was made for AOS-CX 10.02 but ArubaOS-CX 10.03 presentation provides updates regarding new VSX related features too).
Edit: if I'm not mistaken it looks like that ArubaOS-CX Virtual Switching eXtension (VSX) Guide for 10.02 highlighted in a slightly different way the benefits of deploying VSX at core level (for Layer 3) compared to menu of VSX Guide for 10.03.