The Scorpion turreted FSV/MRV was originally designed to be a direct replacement for the Saladin turreted FSV. The Scorpion turreted vehicles designation changed when the Cavalry units decided that they preferred to have reconnaissance vehicles in their Orbat, rather than Fire Support Vehicles. There was also a larger number of the Scorpion turreted vehicles than the Saladin ones. They were intended to become IIRC two per Cavalry troop (out of five vehicles), while the Saladin turreted vehicle served initially (in Vietnam) in a separate troop of 3 Cav. Sqn. There was no difference, except in employment between the vehicles and for a short period when the Army was changing over, they were used interchangeably.
The turreted FV432s were only a handful and they were all issued IIRC to the Berlin Brigade. They carried a Fox armoured car turret. Interestingly, quite late in the FV432's career, there were experiments to improve it's "lethality" and they included mounting a low profile 7.62mm turret on the vehicle. While Volkodov's correct about the difference in the multitude of upgrades/additions which were available for the M113 versus the FV432, the FV432 was actually easier to carry out upgrades/additions to with it's large, circular roof hatch, as against the smaller, square M113s. The designers of the FV432 had obviously thought out matters better than the M113's.