the mAh rating of a batter is its capacity. Like the gas tank in a car can hold 15 gallons or 20 gallons. the amount of load placed on the battery and its capacity determines how long they will last between charges. Just like how a car could get 20mpg or 30mpg.
The only other gotcha there are now low self discharge batteries. All batteries will loose some charge while sitting, its like leaving the gas cap off your car and some gas evaporating. Higher capacity NiMH will self discharge between 10-20% a month or more for some poorly designed (energizer 2500's), you could think of those like having a small leak in the cars gas tank. There are now low self discharge NiMH such as sanyo eneloop, rayovac hybrid, duracell precharged, etc that only loose 1-2% a month. Think of those as havinh a tightly sealed gas tank on the car. The trade off is those low self discharge are slightly lower capacity so they are like having a tightly sealed 15 gallon tank where the high capacity NiMH are like having a 20 gallon gas tank with a leak.
Depending on how you use the batteries you choose the appropriate. a bunch of smaller trips spread out over a month then you can get by with the smaller less leaky tank, if you make all long trips and have to always fill the tank anyway then you could tolerate a bigger but leaky tank since your useing the fuel faster than it leaks out.