I'd give it to Gurlukovich's soldiers.
The hard thing here is to gauge the difference in combat technique between the Fox unit and the mercenaries. How much has combat changed during the 30'something years that have passed between them?
I think the mercenaries have the better techniques, they seemed very skilled when taking the boat, cutting down people from behind without a sound, snapping necks, etc. Very efficient.
FOX... Well, I can't honestly say I've seen much from them. You can take members in Portable Ops, but the stats vary from person to person, I'm not sure.
I'd give the technique edge to the Mercenaries.
Weapons are definitely going to the mercenaries.
The FOX unit soldiers have a Hind-D, and since the two tanks are closer to the mercenaries than the FOX unit I'd say they would seize both. Which means they got two tanks against one chopper. Now I'm no expert on this, can't say what's better.
All in all, I'd say the Mercenaries would take this as far as I can see this at the moment. But if someone brings some new points up that I've missed I might change my mind.
I like these threads. Smart thinking!