A higher mAh rating gives a longer battery life, for example a 2200mAh battery will last twice as long compared to a 1100mAh battery.
An iphone 4S has about 1300mAh IIRC, so you have something to compare with, if you for example get a 1300mAh battery you'll have double the battery time with it, if you get for example a 2600mAh battery you will have 3 times the battery life.. and so on!
So what is best for you is for you to decide as the price may vary greatly on the ones that a lower rated vs the higher rated batteries. Ofcourse the higher the mAh rating the longer the battery life, but if it's any good is up to you. Ofcourse a bigger battery that holds a charge longer will also weigh much more and be bigger if you know what i'm saying.