I'm well aware there's no 10GbE on that board. It doesn't sound like you need it at all however (you couldn't run pfSense at 10GbE speeds with an 1150 setup anyway). I think you also vastly overestimate the CPU power required to do basic firewall/routing/VPN at the kind of speeds one has in a residential setup. A decade old Atom CPU will do it. If you need to use spare hardware, the X10SLM-F will make for a far better router (though still overkill) as it's cheaper to buy and considerably cheaper to run.@BlueFox
Most H3 socket motherboards don't have 10GbE built in. I would say native boards that have 10GbE or Fiber on an H3 socket are less than my fingers on one hand in a semi common form factor.
A home router at my house is not idle in the slightest, Mother-In-Law streams TV just to have background noise I swear... and she's constantly downloading something that she shouldn't. She thinks she needs a VPN to play her games on her iPhone so that she won't get a virus but gets scammed into buying the apps. So power is based off of a processor with load even if its not all the time. Plus Copper is easier to mate to existing networking even though I have a TurboIron 24x switch the fiber patch or copper converter is still 30-40 bucks. 6 copper 10GbE is just right for future proofing and if I buy 4 copper Converters for the 10GbE its about paid for itself to buy the Server board in this thread..
Buying a second one to play with to have one at my parents would just be for learning and setting up Site to Site.
Pitting 4c4t against 4c8T both are close in watts so maybe not a power difference. Both have 8mb L3 which I find interesting wouldn't think a 4c4t would have that cache size. But maybe the 8mb Is standard on most E3 v3's.