I use both methods, depending on what I'm trying to accomplish.
The kick that generates the most "penetration" (I.E., the kind that will "dent" a heavy bag, but not move it, or will break a board) uses the twisting of the hips at the last second to generate tremendous speed at the moment of impact.
Pros: Breaks boards, and by extention, floating ribs.
ultracool) It also transfers a higher amount of Kinetic Energy.
Cons: The chamber and kick itself is more complicated, and can take a fraction of a second longer. Also, the distance has to be judged perfectly to get the maximum potential out of this kind of kick. And, as someone mentioned, twisting the foot after you've placed in on the ground can be tricky in some "real life" applications. (Like when wearing footwear with "sticky" soles.)
The kick that will cause the heavy bag to swing violently, and will throw the guy holding the board across the room, is where you don't pivot the foot, allowing your body weight to lean forward, into the target.
Pros: Is more likely to stop a charging opponent, which makes for a nice defensive kick. It is simpler to execute, which gives you a fraction of a second. This kick transfers more of your Body weight, which means you'll get more of a transfer of Momentum. This kick is more likely to move your opponent, but not to hurt them as much. (I.E. you'll launch them out of the ring, but they won't feel as much pain.)