I would defiantly stick with PCIe for 5 NICs. Additionally Realtek NICs don't offer the best performance and their drivers are hit or miss. The Supermicro board has Intel PCIe NICs onboard and a PCIe expansion slot. This should give you full performance depending on the Atom processor. It really comes down to if you are just moving packets or needing to do packet inspection
The daughterboard I pointed to contains Intel 3 Gigabit chips.
By the way, the OP never told us what would be the intended use for the firewall he needs.