colorex: I thought I read somewhere that a lower C rating made for longer (if less exciting) flight times. must do more research!
The C rating is the rate at which the battery can discharge safely. The actual draw depends on the load you place on the battery.
If you place a tiny motor and prop on a low C battery it will run for a really long time. Same motor and prop on a High C battery and it will run a really long time.
If you place a huge motor and prop on a high C battery it will run for a while at max load and all is well.
If you place that huge motor and prop on a low C battery, it's going to ask for more than the battery can safely provide and it will damage (puff) the battery.
If you aren't asking for a high discharge rate, (glider, transmitter, trainer) then you don't need a high C rating. If you are placing large amp draws on a battery (helicopter, quad, tri, 3D, plane, EDF jet) then you must run a higher C rated battery to safely provide the higher amperage draw.
Hope that makes sense.
Here's an example.
My super cub ran great with the stock brushed motor on the stock 3S 1300 15 C battery. I upgraded it to a Turnigy Park 480 brushless setup. I tried running it on the stock battery at first and it would spin the prop up real fast and then throttle back. The battery simply couldn't provide the amperage to give me full speed. I threw in a 3S 2200 30C from my helicopter and it would not only reach full speed, but hold it for fifteen minutes.
The capacity (mah) of the battery doesn't impact this. The capacity will simply determine how many amps you have available. Your discharge rate determines how fast you can safely use it up. You will never go wrong with a higher C rated battery, but you will pay more for each battery because of it.