I have little knowledge of electronics but I understand that the sensor in the Stopclock reads the light level every 1/100th of 1 second and adjusts the speed of the timer accordingly. Therefore, when the lamp is brighter the clock speeds up and when it's less bright it slows down. I also have the Zone VI compensating timer, somewhere in a box, and frankly it's crap when compared with the Stopclock.
Lee made the point that the Metrolux has shutter time checking and other facilities that the Stopclock has not but I cannot comment on those for I have not used them. Bottom line is that they are both excellent timers and you'll not go wrong with either. I am a great fstop printing fan and until you have tried it you'll never realise the control that it gives. before I used it I was very sceptical but once I started I could never return to linear timing. I take my own fstop timer to all workshops that I do.