they both use the same type of force sensing resistors, invented by Franklin Eventoff. laminated pressure-conductive substrate over an interlocking conductive coil.
i think the 4000 was the last mpc to use interlink. everything since has used sensitronics. mpd used sensitronics, i think, cuz the sensors work with the mpc1000 pad-gaffe correction board...
functionally speaking, they are both the same sensor -- extremely durable, with no moving parts.
pad response has a lot more to do with the AD conversion software (how fast the processor can scan the pad) and the software sensitivity calibration. and, the softness/thickness of the rubber in between your finger and the sensor. and, if there's any air whatsoever in between the sensor and the pad (unbelievably poor design, which i presume was done to cut development costs for implementing note on/off threshold control)