well the first thing is do not aim a kick at someones knee in training or sparring, you will become very unpopular very quickly...
kicking the thigh is fine but don't hit the knee
as far as swivel or step honestly it's more preference/situation then to say one is always superior to the other
in my experience honestly I use both depending on the situation and my opponent
the swivel is quicker and gives less warning to the other guy, but the step is a harder impact