The "C" rating of a battery allows you to determine the maximum current you can safely draw from that battery. By "safely," I mean "without damaging the battery" or "without the battery voltage sagging so low as to unacceptably hurt performance." These can amount to the same thing where lipo batteries are concerned, since pulling too much current will cause the battery voltage to drop too low, which is one thing that can permanently damage a lipo.

To calculate the maximum current a battery can safely provide, multiply its capacity in Amp-hours times its "C" rating.

Using your two examples:

1000 mAh = 1 Ah x 5 = 5A maximum current
500 mAh = 0.5 Ah x 10 = 5A maximum current

So both batteries can safely deliver 5A maximum current. However, the 1000 mAh battery will give you twice the flight time. The 500 mAh will probably weigh less, which could be important in some situations.

NOW a very important point: Most R/Cers do not assume the claimed "C" ratings of batteries are correct. Manufacturers have been known to measure the "C" rating under favorable conditions that do not resemble the way we actually use the batteries. Many experienced R/Cers discount "C" ratings to 80% (or less) of the claimed value.

Thus, a (claimed) 20C battery is treated as a 16C battery in actual use. A (claimed) 15C battery is treated as a 12C battery.

There is no preferred "C" rating, except to say that the higher the "C" rating, the more current you can safely draw from the battery, assuming all else is equal. You must select the best battery for each power system based on the current you need and the flight duration you desire.

In summary, use the "C" rating (suitably discounted) and the capacity (mAh) to determine if a given battery can safely provide the current you need. Use the mAh alone to determine how much flight time you will get.

- Jeff