Stock 64 pitch. The bigger the spur the better. Its easier on the motor to turn the drive train. And a smoother power band from 0 to top speed. Off power you'll have a smoother off power roll very good for stock.
The smaller the pinion in reference to the spur is over all efficiency and less work on the motor. This will also give a wider gearing range. In rubber that will help.
64 vs 48 pitch?
I use 48, The larger teeth will almost last forever in any race condition.
Of coarse now day's spur's are effortless to change.
In stock I would run the 64 pitch.
Hope that helps? I'm not the wizard but maybe he'll show up and help us both.