RE: 4.8V versus 6V for receiver battery?
The 4.8v gives you longer flight time but less servo torque. If you need the added torque go for 6.0v, otherwise the 4.8v would be the better choice. NiCad/NiMH are no difference there (I prefer NiMH as they are more forgiving for charging from partially drained).
If you want longer flights at 4.8v go for a pack with more milliamp hours (maH).