I agree with CG. NIMH batts, will give you a warning about low voltage,with a voltwatch, but also use a meter just in case and a Volt watch.. The Lipo's from what I have read about keep the charge longer, but then rapidly discharge. You do not want that, at least I don't.

Measure the voltage before every flight, just to be sure..

Lipo's, I am not convinced they tell you the correct voltage when nearing the dischage voltage.I don't use them but many people do. You need to also use a regulator type of system when going to Lipo'S. That to me is wasted money and added weight to an aircraft, sure they regulate the volts, but it fails for some reason, now you have more than just a single problem but many, TX?= Batt?=Regulator? =switch?
Keep it simple.Use what you feel comfortable with, but always measure your voltage regardless, for Safety's sake