a Hall effect sensor Ratiometric, you can measure the speed at which the key is struck, "sensitive"
really .. are you planning on measuring the rate of change of the input signal to calculate the velocity.
In the case of a polyphonic keyboard you would need to be doing that for as many keys as you where supporting
Ratiometric simply means that the OP of the device is directly proportional to the input. so the higher the Gauss the higher the OP voltage.http://en.wiktionary.org/wiki/ratiometric
I am interested in how you are going to achieve this with the MUX and the Arduino. It sounds like a challenge .. and i love a challenge