i have a few NiHM batteries that i keep around to let friends use when they drain their batteries. still waiting for one to be returned, LoL.
once you go LiPo, you'll never go back to NiMH. the 1600 mAH lasts about 3-4 hours depending on use. the 1800 mAH lasts the entire day at ACZ (4-5 hours, again, depending on use). that's why i'm replacing the 1600's with 1800's. you can get higher mAH's that lasts longer but i like the nanotechs for their high burst rates (unfortunately, it shortens the run time). you can get a 2150 mAH that will last the entire game with heavy use. but those burst rates are around 30-40c. i'm limited on size on my tavor. 1800 25c is the largest that can fit. these are all 7.4v, 11.1v requires stronger pinion and piston.
browse the LiPo's on Hobbyking for the size that fits in your AEG. you can stick with xt60 but i already have a bulk pack of dean clones. also, don't forget a mosfet (electioneer/Reed sells em locally or get an AWS microfet). i can wire it up for you. balance chargers are also recommended. i have a cheapo that charges 5x 2S packs (7.4v) at a time. they require an external power source.
avoid tamiyas, LiPo's don't like them. when tamiya's loosen up, they become prone to connector meltdowns.