with all that replies, you probably have enough informations to help you choose the good battery.
I would suggest you to go with NIMH batteries for the reasons already posted. And get a good battery charger like this one B3 Charger
. You can also get more "Intelligent" charger but the price will get high rapidly.
the 8.4V 3000mAh will add some ROF to you AEG and will get you more capacity and will last longer than a 1600mAh. I forgot to mention that the 3000mAh is bigger in size so check and make sure you have enough room to store it in.