Strike to the neck > strike to the groin. I've kicked someone in the balls in a real fight, and it didn't stop them.
Yeah, when things are REALLY serious, a general kick to the groin usually just pisses people off. They kick your ass, then worry about their sore nuts later.
The one exception - ANY time - is if you land squarely on a nut and rupture it. Then they go down, no questions asked.