Only Peking University and Chinese Academy of Sciences are first-class in the field of mathematics in China. However, there is vicious competition between the two schools, which is a bit typical (China's international status in mathematics is low). Tsinghua is good at engineering, but not as good as those two in math. Both of them are fine. Not even, let's see what other conditions are more suitable for you. And you can also consider which direction you are more focused on. Analysis? Algebra? Topology? Or application? Everyone has his own strengths!