? Q on battery run time

[digger]

i have a Orca pro spec VC4 MKII ESC with a orca 540 7,5t motor

my genral Q is i have a choice of two batteries

1/ Ansmann Race Xirius 7.4v 4000mAh 20C LiPo Saddle Packs

2/ Team orion rocket stick pack 7.2 - 5100mAh

Just working on the two batteries above what would be the best to buy for club racing

what sort of run time would i be looking at , i know this depends on driving style but im new to the electric side of things and just would like to know an average run time
Thanks Digger