This is way more than you'll need to know for airsoft but good to know anyways:
http://batteryuniversity.com/learn/
tl;dr Lipo is lighter, more compact, and has more energy density.
A 7.4V 1500 mAh lipo for example is way more compact than an 8.4V 1500 mAh "stick" or nunchuck or whatever (made up of a bunch of 1.2V 1500 mAh Sub-C batteries or whatever).