I recently worked with a 10-year old girl who was ranked very high in the junior standings and was considered an up-and-coming star. Before prescribing strength exercises and skill-specific activities for her to engage in, her strokes were first analyzed to determine which exercises would be of most benefit to her.
In so doing, several major stroke execution flaws were uncovered. For example, she did not make effective use of the hips or shoulders, and her swing was essentially executed with the arm. Most often she did not step into the ball, did not get into an effective side-facing position, and hit many balls from a flat-footed position.
However, her shot production was fairly good. She related that she and her coach were working on many specialty shots and that she was mastering them. But, learning to master a shot or to hit a ball in a particular spot from a particular place on the court is extremely difficult if your stroke is not as effective as it could be.
The parents were inclined to agree, but they did not give it high priority. As a result, only her speed and quickness were worked on since any corrections made in her strokes were not followed up on and her practices were devoted to making shots and playing strategy. But by being able to execute the groundstrokes more effectively, she would have had less trouble mastering specific shots.
Thus to answer the question in the title of this article should be a resounding stroke first before executing the shots. Effective execution of the strokes especially at an early age, is much more important than any specific shots. When youngsters develop the ability to execute effective strokes they will have the ability to execute any and all shots at a later time. This is will be the key to their success.
For more information see Explosive Tennis: the Forehand and Explosive Tennis: The Backhand. It is available as an e-book on a CD.