pivot speed meter has just the basics of taking off speed cut and giving u digital speed readings... and also calculate 0-100,200,300,400 meters.
RSM has waaaaaaaay more functions... batt voltage, rpm, RPM shift light, speed warning, speed, 0-XXX meters, 0-XXX km/h, some memory space for u to save ur timed readings, as well as display top speed, got real time data logging for forgot how many seconds... etc etc etc... !! =Þ
Again... work with your budget... if ur budget allows, RSM is quite a cool toy to play wif... i used to have one. =Þ Sold it off coz' my car got broken into and almost gotten my stuffs stolen (my alarm rang, i went to check the car... and saw the guy run off).