C stands for capacity and the number is a multiplier and in this case it is describing the discharge rating (probably constant current MAX)

Theoretically the cell can be discharged at 30 x 1.3 ah = 39 amps and this would equate to the cell being drained in 2 minutes. Your chipmunk won;t use it up that fast so a 20C or a 30C is just fine. The 30C having just a slight power advantage due to it carrying a higher voltage than the 20C. It won't affect flight time any.