It's too close to call. Given that the 'C' ratings are about as accurate as the capacity and voltage numbers on NiMh cells (ie, not very!) then they have to be taken with a pinch of salt anyway. If you are running a 10.5 motor which is probably 250W at best, then (W=VxA) you aren't going to draw more than 80 amps continuously, so having (50x6) 300 amps available is rather academic! Which leaves us with the capacity.
Those numbers are usually more accurate, and they do get checked by people like ROAR and BRCA as part of their testing for homologation. Since most people can check this on their chargers too, the manufacturers tend to be less *cough* imaginative with these numbers. The more capacity you have, the less voltage drop you get during the race. The less voltage drop you get, the higher the performance towards the end of the race. If you are going to run in the new 10.5 Stock class with no timing boost, this will become essential.
Given those cells, I would choose the CRC 6300 cell. HTH
