Think of it this way:
If electricity would be a river, the width of that river would be the voltage, while the flow of water would be the Amps.
More Voltage will allow for more electricity to get to your motor at the same time, and would make it spin faster under low load (forget the water analogy here, a waterwheel would not be the best of example, I now realize :P)
More Amps will be drawn by the motor from the battery to make it spin harder/stronger. This is what you want more in your case.
But don't confuse mAh and Amperage, or more precisely, discharge rate of the battery (higher discharge rate means more current delivered at once).
mAh's (milli ampers per hour) will reflect the amount of electricity the battery hold. A 7.4V 1000 mAh battery can have a low or high discharge rate regardless. It will just last longer with a low discharge rate than a higher one.
So to put it shortly;
NiCd battery (nickel cadmium): old technology, you don't want this. heavy, not efficient.
NiMh battery (Nick Metal Hydride): Good compromise between efficiency, price, and bulk. Usually round like CR123, AA (like the NiCd too). Usually the discharge rate of these batteries is not mentioned anywhere. You'll only have Voltage and mAh's rating. I'm not sure if it is just because, or if all NiMh are considered to have the same discharge rate.
Lipo/LiFe battery (Lithium Polymer / Lithium Iron, and other lithium based batteries):
More modern tech, will usually be more expensive but also much more efficient for similar sized batteries (compared to NiMh). Usually Square-shaped wrapped in some black plastic.
Needs to be handled with care, especially the LiPo's, while they're still comon and more and more widely adopted despite the "risks" they involve (lots of threads on ASC already)
Usualy requires a attery monitor of MosFet installed on the gun, and be sure to have a good charger especially made to handle Lithium-based batteries.
Regardless of that battery you buy, get a good charger like the iMax B6 or similar. IT will allow you to keep your batteries longer, and one charger will be able to charge most if not all types of batteries if you decide to change in the future.
Hope this help clear a few questions you might have. Note that I am no specialist on the matter and might be wrong or mistaken here and there, others will probably correct me the case being.
Originally Posted by Brian McIlmoyle
Real life comparison,
GBBR- bang bang -- Giggle
AEG-- merrrzip merrzip -- meh