Differences and Benefits Between i440fx and q35 in Proxmox

My view

For application i440fx is enough, and it is simple, for hardware related, such as GPU passthru, then Q35 is better.

Switch between them

After switch between, the network interface name will be changed.

Q35

Q35 supports PCIe

• Extended configuration space (MMCFG)
• PCIe native hotplug
• Advanced Error Reporting (AER)
• Alternative Routing-ID Interpretation (ARI)
• Native Power Management
• Function Level Reset (FLR)
• Address Translation Services (ATS)
• AHCI storage controller
• vIOMMU emulation
• Secure Boot

Q35 limitations

• No support for legacy guests (Windows XP/2000).
• Questionable support for legacy QEMU devices.
• Limited IO space can affect the number of devices used by a single Q35 machine

References

Differences/benefits between i440fx and q35 chipsets?
Q35 - QEMU
PCI vs PCI Express
PCI EXPRESS GUIDELINES