You want to make sure you chose a laptop battery that use the latest advancements in li-ion cell design. The newer the li-ion cell design, the higher the capacity per cell and the longer the service life.
Basically, the best cell made today are the Panasonic NNP li-ion cells each cell stores 2900 mAh of power vs a typical 2200mAh... this is about %30 more power and will last for 3 Years . But the cells are costly, so don't expect to find them on Amazon.
A 6 cell battery made with these cells would boasts specs of 5800mAh / 62Wh and a 9 cell would be 8700mAh / 94 Wh.
The Max Capacity
brand exclusively uses these cells and I'm very happy with the performnce. I bought mine for about $72 from SafeBatteries.com