There's another differance Pot Ferri/Br will lighten the highlights faster than the shadows, so is often used for slit toning. while Farmers reducer will lighten the shadows more which is what it was often used for.

Beware versions of Farmers Reducer using Rapid fixer they aren't Farmers reducer at all and work far to fast. Farmers reducer uses plain Sodium Thiosulphate and Potassium Ferricyanide.