A cells State of Charge (SOC) will determine its capability to hold voltage under load.
A cells overall State of Health (SOH) will also play a factor typically as well (effects due to number of charge cycles, rate of charge, number of deep discharges, cell temperatures, etc).
Power (Watts) does = V * I, but if the motor draws 20A the 5000mAH 25C and the 65C battery (of same voltage) will produce the same amount of overall power.
For racing (like VTA), the battery capacity is what I'm interested in most. I want to stay in the SOC sweet spot as long as possible. This means my average power throughout the 8 minute race will be greater and maybe help me win that drag race to the finish line (assuming I drove perfectly for 7:59)
.