Your situation is exactly like mine. I've a Mac Pro 2010 and planning to get a used Mac Pro 2013. However, my existing Mac Pro 2010 has a lot of PCIe-devices connected. Such as Vega 56 and bunch of external SAS RAID drives + SAS enclosure (to house those SAS RAID drives).
If I switch to Mac Pro 2013. Then I also need to get one external SAS-to-thunderbolt enclosure to house the SAS controller. If I switch to external thunderbolt-based enclosure to house those external SAS drives. I would also need to invest the very same amount of money. In either case, I would need to buy a Mac Pro 2013 + SAS-to-Thunderbolt enclosure at the same time. Which is $$$ + $$$$$.
I'm still thinking alternative. My Mac Pro 2010 is very very fast when processing 4K video footage on FCPX. The only reason is the OS upgrade capability to newer macOSes in fore-coming years. It seems that to pay for double hardware for such an upgrade is silly.
If I switch to Mac Pro 2013. Then I also need to get one external SAS-to-thunderbolt enclosure to house the SAS controller. If I switch to external thunderbolt-based enclosure to house those external SAS drives. I would also need to invest the very same amount of money. In either case, I would need to buy a Mac Pro 2013 + SAS-to-Thunderbolt enclosure at the same time. Which is $$$ + $$$$$.
I'm still thinking alternative. My Mac Pro 2010 is very very fast when processing 4K video footage on FCPX. The only reason is the OS upgrade capability to newer macOSes in fore-coming years. It seems that to pay for double hardware for such an upgrade is silly.