I use a 6V 1,400mAh NiMh in both my 50cc Extra and Edge. Was using a 1,000mAh NiCd in my EP Venus II, but going to try a 700mAh NiMh next time I fly it (but I'm not running digital servos in the Venus II).

If you're skimping on battery capacity you should check the Voltage before every flight. Be certain to bring a field charger with you.