For most applications, 48p is fine, however, 64p allows for more exact gear ratios. 64p gears are really necessary for pan cars when back in the day people were using nimhs, brushed motors, and had to really achieve exact gear ratios to make a 4-5 minute race, taking into account battery power, motor, and tire diameter of their foam tires. Foam tires change diameter practically after every run, so back in the day, exact gear ratios really mattered because nimh batteries didn't last as long or provide as much power as lipos.
The only advantage to 64p over 48p is being able to achieve a more exact gear ratio.