I put this thread up because the answer is the typical egg before chicken question.
It can in fact be argued both ways.
A quality response is important just as a quality movement is.
Dressage should be such that nothing is so set that there is only one answer. Good riding, in my case dressage but can apply to any discipline should carry no absolutes. Horses are unique and training methods must follow that uniqueness. Valid arguments can be waged on both sides.
I had an answer to this question with this response.
Because the answer is self evident, i.e. OBVIOUS. So it's not really much up for discussion.
All you can expect the first time a horse learns a movement, assuming it has been well prepared, is a response. You can then work on the quality of the response. All the time, though, you have to keep your eye on the quality of gaits because if you lose the quality of the underlying gait during the movement, something is wrong with your preparation.
For example, when I taught my mare the shoulder in, I did not expect the degree of angle or bend that is required in a show and I did not expect the quality of expression. I just wanted her to respond to my aids and also to retain the basics of rythm and relaxation and the quality of the gait she had at the time. Then you work on angle and bend and expression with various exercises. I have been working on shoulder in for over a year now and we are now adding expression by doing tempo changes and transitions in the shoulder in which helps to strengthen her. The quality of my shoulder ins, after a year of working on them, is much higher than I could ever expect initially. She is doing so well (I worked on lateral suppleness so that I could do changes of bend) that she can now do the change of hand down centerline in shoulder in and she can do it with quality of gaits. Also as a horse develops, the quality of gaits can improve (especially the trot), and this can improve the quality of the movement as well.
So, IMO, the answer to the question is obvious. The response is important first (while retaining the basics and quality of gaits) and the quality of the movement improves with time and proper gymnastics.
And here is my arguement for the reverse--movement.
The better the movement, the better the response will be AND the better the response the better the movement.
The horse will not respond quick or proper until the rider has set the horse up in a balanced and engaged manner, this is movement. There can be no response good enough for the horse to be able react physically correct without the quality of movement being there.
Yes you can get some semblence of responce in a shoulder in with dragging legs and call that responce. There are many that train this way. Then there are the purests that insist in a pure forward movement correct in every way before they ask for the tinniest of baby step of a shoulder in.
For instance, if the horse is not in front of your leg, he can't respond as quickly or effectively. Being behind the leg and therefore not engaged cannot result in quality movement, and therefore response.
So it makes no sense for example in asking for leg yield if you don't have a relaxed, balanced and forward horse, that is rythymic and supple. So, you need quality of movement BEFORE asking for lateral work.
In summary: If the response is there, the movement will be there. And, if the movement is there, the response will be there.
So in fact both answers can be considered somewhat correct and the moral of this thread is that there simply are no absolutes in any discipline even in a structured one like dressage.