Both methods of solving for lengths of a right triangle are effective. That being said, the Pythagorean theorem is well-known and only requires a student to know one equation (as well as where the legs/hypotenuse should be plugged in). The six trigonometric ratios are also helpful, and over time are something that students begin to recognize when looking at right triangles.
I believe that the Pythagorean theorem is more effective because of its simplicity and wide application. The six trigonometric ratios will not apply to every right triangle, and require more memorization.
Hope this helps!