I usually tend to think of several major kicks mostly in terms of how the weapon is delivered
1) Snap (front) kick
2) Roundhouse Kick
3) Side Kick
4) Back Kick
This is just a general idea. Such kicks as cresent kick or hook kick could possibly be considered to fit in f of the above categories...or that might be stretching it.
Most of these kicks can then be modified in several ways. The first point of modification is whether you are delivering it wth the front or rear leg (assuming normal fighting stance). The other major modifiers are jumping and spinning. Another set of modifiers is any footwork you use to a) deceive or b) change distance. This includes switch stance, step behind, step in front, slide step, hop-to,etc...
So for example, a sidekick can be delivered as
1) Rear leg
2) Front leg
3) Front leg (Hop to)
4) Step behind
5) Step in front
6) Spinning
7) Jumping
8) Jumping & Spinning
9) etc....
Between two instructors I've learned two kinds of switch stance. One is a complete switch, including rotating the shoulders, so that you are in the opposite stance. The other is a quick foot switch from the waist down but without the shoulder movement. I'm finding the first one sets up a spin kick well if I can do it fast while the opponent is adjusting to my stance change. The second is effective to bring your front leg back to kick with what was originally your front leg but with a bit more torque (and deception).
What was my point? Oh yeah, just agreeing with Miles that there are *lots* of variations to basic kicks. Different variations work better for different situations, but you should practice the ones you are going to use.
I've never used a step-in-front in sparring, but it seems to me that if you are in a standard fighting stance, right leg back, and want to throw a step-in-front that your rear leg is going to go straightforward toward the target and then your kicking foot is going to come up and fire at the target while your shoulders/hips rotate into position. A step-behind requires you to bring your rear leg over a bit so that it can step behind your kicking foot. At that point, your hips and shoulders are aligned for the kick, but you have to get in that position which either ends up telegraphing a bit or you can do it if you've naturally ended up a bit skew from your opponent. Coming from limited experience I don't see how either is really more powerful, they just seem to be differenet ways of using our footwork in response to your opponents positioning