the 1200 or 1500 is the capacity of the battery. it is measure in current over time or mAH or milli ampere hours. there are two reasons why higher capacity is better:

1. length of flight - meaning that if your set up draws 5 amps (5000mAH) you can fly it for 1200/5000 hours or about 15 minutes on the 1200 battery. with the 1500 you could fly for 1500/5000 hours or about 20 minutes.

2. discharge rate - when you run your motor, it discharges the battery. depending on the lipo, it could have different maximum discharge rate which is generally based on the capacity of the battery, commonly referred to as C. therefore if you have batteries with the same discharge rate, which is measured in C's, you will be able to use more current (producing more watts of power) with the higher capacity battery. so lets say that both the 1200 and the 1500 have maximum discharge rates of 10C. this means that with the 1200 battery, at full throttle, you should only load it to 12 amps and no higher. with the 1500, you can pick a setup that pulls 15 amps at full throttle.

of course there are two considerations against the larger capacity. one is money and the other is weight. higher capacity batteries generally weigh more and cost more than lower capacity counterparts.

hope that answers your question