RE: 4.8V versus 6V for receiver battery?
Charlie P thats BS.
The only difference between a 4.8 volt and 6 volt batterie is the six volt gives you more torque. (mostly for bigger servos)
As far as how much time you have till the batterie is dead, that depends on the maH rating. ( The higher the maH rating the longer the batterie lasts)
An the difference between a NiCd and a NiMh Is that the NiMh usually has, on average, a higher maH rating. And they are much more forgiveing when charging partially charged. And NiMh are slightly heavier.
These are the main differences. Good luck
Electrolight