Why would it have to be a 3 CHess piece arena? Two countries = Sabre vs F@got= no need for perks.
I have to side with Karnak on his response. If it was a 2 country system, I get the feeling most WOULD be flying Allied for the F-86.
THIS IS WORLD WAR II!!! NOT KOREA!!! 
Oh? We have a WW1 arena... Still not seeing why we shouldn't venture into the Korean Era, AFTER the WW2 line is filled.
KarnakI don't think the F-86 would lord over the MiG-15 as much as you think. US pilots were better trained than their Korean counterparts, most of which were Chinese/Korean. Soviets were better trained than they were and probably on par with US pilots. Now it being chosen because it's American, you won't get an argument from me. We all know that alone is the reason a lot will choose it. But I am more than certain that you will have dedicated MiG-15 pilots. On a side note, I didn't know the Yak-9U was that much better at high alt than the 51...

I'm a bit serious. I've only flown the Yak-9U 3+ times since I started AH in Sept-Aug of 09. And I've only flown the 51 as Jabo, so my experience with these planes are rather limited.
Hopefully someone with books on these 2 planes will come in here and post detailed info about them. Regardless, I'm still +1 on a Korean Era Arena down the line.