RE: 4.8V versus 6V for receiver battery?
I use 5-cell packs in planes that have high-torque servos, I figure I might as well squeeze all the torque I can out of them! I've also heard of guys using 5-cell packs because you'll still have four good cells if one cell fails (they usually short out when they fail), and thus have plenty of voltage for your receiver.
As far as which one lasts longer, I've heard arguments both ways. You do get more power out of the servos with a 5 cell pack, but then you have more power in stored in a 5 cell pack. Remember, power is in Watts, not Amps. With 2000mAh cells, a four cell pack has 9.6 Watt/hrs of power stored, and a five cell pack has 12 Watt/hrs of power stored. This means you can put out 25% more power with a five cell pack and it will still last as long as a four cell pack (with identical cells).