Depending on which aspects of the Middle Ages you might be analyzing or looking at, this period of time in Europe can be viewed as both a prosperity and a decline. However, from my personal opinion, I would say that they were more of a prosperity if I had to choose only one.
The Middle Ages are marked by the fall of the Western Roman Empire, which is why the dawning of the Middle Ages may be viewed as a decline. Throughout the Middle Ages, however, there were many improvements upon society and beneficial discoveries that would lead one to interpret the Middle Ages as more of a prosperity.
During the Middle Ages, middle and western Europe were greatly urbanized. Because of this, many new countries were 'born.' This is, in itself, a prosperous account for Europe. This was also a time of vast expansion for Europeans overseas.
In terms of art, the Middle Ages was the rise of humanism. Humanism as an art form was extremely progressive. Humanism was shown in art by taking a more realistic, rational view as an artist, showing raw human condition in artistic works.
Architecture at the time was viewed as 'Romanesque.' Ornate, well-adorned churches and buildings made of rich stone were often so well-jeweled that they were robbed. This style of architecture brought back a grace of Roman architecture that may have otherwise never been resurrected. This is another leap artistically during the Middle Ages for Europe.