It's to do with how much current they will give.
xxC x capacity gives you the current the battery can give.
20c x 4.9 (4900mAh) = 98 amps
25x c 4.9 = 122.5 amps
Jimmy lent me a trakpower 4900 stick and 3200 stick to play with at Southport last week, the 3200 had a lot less punch than the 4900 - even with a 7.5 in 2wd and it being a lot lighter.
:::EDIT::: Just beat me to it Chris
The 3200 (not sure if it was a 20 or 25c, Jimmy!) felt very similar to a NiMh pack to drive.