The shin is definitely more powerful. Make sure you're kicking right (that is if you're doing Muay Thai). The power doesn't come from a snap of the leg, but from the violent twisting of the hips.
Go up to a heavybag. Kick like I just explained. First hit with your foot. The foot makes a loud slapping noise. Sounds pretty powerful right? Now connect with your shin, and really dig your shinbone in. You won't get that same thunder-like SLAP! that you got from your foot, but if you're kicking right, you should have seen the bag fold or jump in some way. THAT is power.
Put it this way. Everyone knows that on a baseball bat, there is a "sweet spot." Some bats even come with a red line painted on this sweet spot. It's the point of the bat where you can generate the best power for a home run. Now where is the sweet spot located? It's not at the end of the bat. It's more towards the center of the thick part. You should think of your legs as baseball bats. Use the "sweet spot" of your legs (your lower shin), and drive a home run through your target with violent rotational force.
Whenever I kick my Thai bag, it immediately becomes apparent that the shin is just overall better than the foot. First off, if I'm not connecting with my shin, and more with my foot, my toes and feet start to really hurt (I own a TKO 125 pound Thai bag; the thing is stuffed SOLID as a rock; my shins used to bruise like crazy whenever I hit it). When I kick with my shins, obviously, I save my feet. Not only that though, but when I connect with my feet, I barely move the bag. I make it swing, but you should be focusing on folding the bag, not moving it. Now when I hit with my SHIN, I watch this behemoth bag start to fold, and it even "jumps" up a little bit. That's the point where my family starts to complain that the house is shaking.
Here's another thing. Basic science tells us that the more dense an object is, the less it disperses force when it strikes something. What hurts more to get hit with (if the objects travel at the same speed), a 16 ounce glove or a 1 pound rock? The harder an object is, the less energy wasted when it transfers energy to another object. This is one of the big reasons proper bone alignment in a punch is important. If your wrist buckles when you punch something, not only do you hurt your wrist, but the energy isn't entirely dispersed into the bag or opponent. If you hit a bag with your fist unclenched, and then hit it with it clenched, which is more powerful?
Likewise, the less moving parts on a weapon, the better it is. Your foot is held onto your leg by your ankle. The ankle is movable. So when you hit the bag, your ankle will most likely move in at least some small way. That's wasted kinetic energy. Now think of the shin. It's one big, solid piece of bone. You hit something with that, and you can guarantee that the receiving end of a blow from the shin will take all of the force of that strike. The shin is sharp, solid, and hard.
That's why Muay Thai kicks are so much more powerful than traditional martial art "snap" roundhouse kicks. Traditional artists use the muscle of the quadricep to power a snap of the knee, and connect with the foot. Now, compare that to using the power from your core (the center of all athletic movements and power), generating extreme rotational force, and connecting with the solid, sharp, and unyielding shin.
