When using tweens between frames in Flash, you can either use the motion tween or the shape tween. Both have their positives and negatives, but both also have their uses. For me, the motion tween is the one used most often.
Motion tween works with movie clips and you can only have one single component on the frames you are tweening to not get an error. This is great to use if you want to show an item moving, rotating, changing in size, or a combination of any of these. Beyond these uses, the motion tween is helpless.
The shape tween on the other hand deals with items that aren’t made into clips. It can use any amount of drawn items on the stage during its tween and can change the shape and color of these items. It can also make things move, rotate, and change in size, but not in the precise way that the motion tween does. There are many times when the shape tween will give you unexpected results and oftentimes you will need to give it help to get it to morph in the way that you want it to. It calculates the shape shifting itself and always seems to take the quickest path to the new shape, even though that may not be what you want. The shape tween also takes more effort on the computer’s part to play when it is finished and on the Internet, though this is becoming less of an issue as computers are becoming more powerful.
Both tweens have their strengths and weaknesses and do what the other cannot. You just need to decide what you want done to choose your tween. But for the most part, motion tweens get the majority of things done without any headaches.