Both are pretty simple and sound self defense techniques. I give a whole lesson concerning intimidation, etc for a front two hand lapel grab. Knee of Vengeance works well for a static grab of intimidation or a pulling grab but not a shoving/pushing grab. It also would be considered an illegal technique in PA because the grab is a simply summary offense and the response could easily be considered aggravated assault.
Fang of the Cobra works well for a static front choke, a pulling front choke and a pushing front choke. It, on the surface, is not an illegal response because a front choke can be lethal. The good news is that I've run into few men who really know how to set the choke properly.
The two techniques are fairly interchangable.