I think MMA fighters hold the advantage. Just because we don't see them throwing kicks to the groin or using eye gouges when it hits the mat, doesn't imply they don't know how to.
I'm sure during the course of their years of training they have learnt many "illegal" underhand tactics that could be used in a "street fight" fight to the death simulation. They don't use it in the organization they fight in simply because they are not allowed. And as such they are probably aware of how to defend things, a kick to the groin probably serves as a kick to the midsection and as such the same defence would be used.
That said Commandos and Marines are tough sons of bitches and know how to fight, however I would put my money on the professional MMA fighter.
Agreed with the shaolin monks, tv films and literature have all made them seem invincible. They are able to perform great feats and are great martial artists. They ARE NOT great fighters. As ironman said, they have been destroyed vs muai-thay fighters in the past. At the end of the day my money goes on the MMA fighter.