I always preferred having a slight bend in the knee and having virtually no snap in the kick. It was the first way I was taught so I am probably biased, but I always felt that it had the most power and I could keep my arms in a guarded position when I threw. My current instructor likes me to chamber and snap the kick whilst bring the arm swinging down. I don't like this method but I do it anyway. It is a bit faster and more balanced though. In muay chaiya, the kick is done with a very bent knee though this is more because you are fighting in closer and the kick pushes the opponent out so you can flow your other attacks off it.