mAh=milliamp-hours, which is how long the battery will last

C rating is how fast the battery will discharge.

If you're doing only speed runs then the more important number is the C rating. However as you get to better batteries the mAh and C ratings will both go up.

You may want to consider quality too. A cheap battery with a high c rate and mAh may do what you want, but not last as long as a better battery.