UFC guy, hands down.
The special forces guys are weapons/assault/stealth specialist. There self defense is 10% of their training, while a UFC guy spends all day working on hand to hand combat.
The people talking about Krav Maga don't understand that it is designed to be used against a untrained opponent. Much of Krava Maga is similar to Kempo. It is designed to hit an un-ready opponent multiple times quickly and get space to run away (or fire your weapon) or to dis-arm them and level the playing field. It is not designed for fair, hand to hand combat.
A well trained MMA guy is going to have an effective jab that keeps all those kempo or krav maga moves out beyond range and/ro is going to have effective take down defense.
There is a reason Chuck liddel doesn't use any of the kempo strikes in the ring.
Bottom line is that unless the special forces guy has spent a decent amount of extra curricular time studying boxing, he would be in deep trouble quickly and likely on his back before he knew it.
Ignoring the fact that the special forces guys aren't really trained in martial arts the way MMA guys or even top krava maga guys are trained, there is debate in the martial arts community over, the "sport" MA and the "too deadly" or "real life" martial arts, and frankly it really seems to be that the "sport" guys have won. Krava Maga is kind of cross between the two and therefore has gained quite a bit of popularity as a practical self defense art.
What appears to be the case is that real fights are too disorganized and the "real life" martial artists are to un-used to real life combat to take advantage their training against another well trained fighter. Bottom line is that it doesn't matter if you have some supper throat chop, eye gouge, finger wrench, if you are un-prepared to take a quick uppercut on the way in you are on you back because of a "sport" strike and somebody is now stomping on your head.