The only difference between upper and lower pickups is the size. They are wound with the same wire gauge and number of turns and use the same magnet. The lower pickup is longer and has a lower resonant frequency, but this is not likely to influence the sound in any way as the resonance is well out of the audio band. Any difference in sound is due to the fact that the lower pickup is placed at an angle to the strings and is sensing them at different locations than the upper pickup.