I have been flying large heli for many years and just now building a trainer nitro plane, and you probably have settled on a decision.

I will say this, I have never had good luck with nimh batts they self discharge and generally have been unreliable in the helis. BECs from the motor to the radio servos ( electric powered helis) also can fail dropping volts to the RX in flight.

A123 batts are my best option and will power all my helis and future planes here is some good info on them.

http://hangtimes.com/a123_batteries_for_giants_faq.html