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).