ORIGINAL: Missileman

Nimh has more capacity but can't provide sustained high current draw (most servos are ok but big electric motors may be too much for an Nimh)
That said, I am running a 6 volt 2,000 mah Nimh made by Hobbico ($29 at Tower) on my 1/4 scale RV-4 and despite its 7 servos I can still fly all day with no sign of a low battery.