Nowadays, many couples have only one child. I believe the disadvantages of this trend slightly outweigh the advantages, although much depends on how the child is raised.
There are clear benefits for both the child and the parents. Parents can devote their time, attention and money to one child, providing a good education, a comfortable home and more activities than would be possible with several children. Only children often develop strong language skills and independence, as they spend much time with adults and learn to entertain themselves. The family may also experience less financial strain, and parents with one child can continue working, travel more easily and maintain their own interests, which is attractive in expensive cities.
On the other hand, there are important disadvantages. Children without siblings miss the daily experience of sharing, arguing and making up, which teaches negotiation and cooperation. Without brothers or sisters, they may feel lonely, especially if their parents work long hours, and they may become spoiled, accustomed to being the centre of attention. They also carry the full weight of their parents' expectations, which can be stressful. In later life, an only child has nobody to share the responsibility of caring for ageing parents, and may feel alone when the parents die. At the national level, widespread one-child families lead to an ageing population, as seen in China and South Korea, where the workforce is shrinking.
Parents of only children can reduce these problems by arranging regular contact with cousins and friends, and encouraging team activities at school.
In conclusion, having one child offers financial and educational advantages, but the loss of sibling relationships and the social consequences make me believe that the drawbacks are slightly greater.