Peking University is a comprehensive university, and its key disciplines are basic disciplines such as liberal arts and science. Therefore, having a strong scientific research ability will have an advantage in various rankings. But relatively speaking, when looking for a job, the basic subjects will be almost worse. In addition, Peking University is more symbolic politically, and foreign leaders often give speeches at Peking University when they come to Beijing, so its international reputation is also higher.

Tsinghua is an engineering college, and its key disciplines are applied engineering such as electronics, computers and architecture. These subjects are easier to find a job. However, because it is also an applied discipline, the scientific research nature of engineering is relatively weak, and the scientific research strength of Tsinghua is not as good as that of Peking University.

Another reason may be that Tsinghua's name sounds more powerful.

Besides, you'd better be modest and give your mother a little face.