It is really more of a drivers preference if you want to run one or not. Basically using a shotky diode on the motor will increase braking. Depending on the track conditions, I have run up to 4 shotky diodes on my motors. It wont hurt anything if you decide not to run it though.
If you dont run a shottky diode you might overheat and damage your brake transistors in your speedo.
If you are running a GM speedo you must use a shottky, if you dont your brake will be gone faaast.. (did it to 2 gm speedos..never again)
They are all the same, just buy the brand that offers the cheapest diodes...
GM has a stronger type , looks like a transistor. Have never tried it myself, but i have heard that its good (over 2 times stronger than a standard shottky)