The higher the C rating the more amps the battery can handle and is capable of delivering. Take those batteries you mentioned: 2100mah 15C - divide the 2100mah by 1000 to get amps = 2.1, then multiply that times the C rating = 2.1x15=31.5 amps that the battery can deliver under a continuous load. The second C rating number, 24 in this case, is the max. 15 second burst rating for the pack.

The difference between a 15C pack and a 25C pack is that the 25C pack can handle and is capable of more amp draw, that's it. If they are both operating under the same amp draw, they will drain down the same. The C rating has no impact on how fast the battery drains. Just how much the battery can handle.