Two different parameters here.
The first, 2000/3000 mah. This figure is the battery capacity. It's how much energy the battery can store and allow you to use before being recharged. Obviously the one with the higher mah rating will give you more battery life.
The second, 20/30 amp. This figure is the current you can safely draw from the battery continuously. This figure is the one you need to worry about to make sure you don't exceed it since excess current draw heats the battery quickly.
If I draw 15 amps (1500 ma) from a 2000 mah battery, the battery theoretically will last for 1.33 hours.
If I draw 15 amps from a 3000 mah battery, the battery should last for 2 hours.