These two uranium isotopes decay at different rates. The half-life of the uranium-238 to lead-206 is 4.47 billion years.
The uranium-235 to lead-207 decay series is marked by a half-life of 704 million years.
Uranium is not the only isotope that can be used to date rocks; we do see additional methods of radiometric dating based on the decay of different isotopes.
For example, with potassium-argon dating, we can tell the age of materials that contain potassium because we know that potassium-40 decays into argon-40 with a half-life of 1.3 billion years.
Radiometric dating, or radioactive dating as it is sometimes called, is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes.