It is quite normal for parents want to have one boy and one girl. However, if you mean by sex selection, getting the mother to abort a female fetus, it is unethical. Some people adopt a child of the other sex, to take care of this issue. But that is also not a very easy option.
A lot of people believe that only children tend to get lonely and that having a sister or brother makes it more pleasant in every way. This is a matter for the parents to decide. I have met only children who are sociable and are close to cousins and school friends and also only children who are spoilt and self-indulgent. I have come across persons who, as children, had to share everything and resented it, or were disgusted with always being compared to a sibling. I have also seen families, which have a lot of fun with two or more children, and there is a strong brother-sister bond. In real life, there is no single or simple formula that works for everyone.
I think it is wise on your part to consider your financial and family assets. Here again, affordability is a matter of perspective. Also, the mother may have to take off from work for two or three years, if you plan
another child. Many young couples seem to depend on the grandparents to take the responsibilities of child care, but there is no guarantee that they will be continuously available or have the energy for the task.