Battery choice should be determined by the amp draw of your motor

Decide on a motor

Find out the burst amp draw

Get batteries that are able to supply a constant rate equal to the burst rate of your motor

Example
Motor with 100 Amp Burst
Option 1 - 5000mah 20c pack = 100 amps
Option 2 - 5000mah 30c pack = 150 amps

Option 2 is a better choice as option 1 is borderline

Generally speaking packs that have a higher discharge 'c' rating will hold a higher voltage under load

More volts = more speed

Cheers