RE: 4.8V versus 6V for receiver battery?
The key factor here is "of equal capacity" or words to that effect. Equal capacity means SAME mah ratings. All Charlie P (and Futaba) said is absolutely correct.
Like anything else, if you want more capacity, well, increase it!! Rocket science, I guess. Want longer flight times? Put a larger fuel tank in the plane. If you want longer battery duration, put a higher capacity battery on board. Simple as that.
When I went from 4.8 to 6 volt, I increased the capacity by about 20% to 25%. I get just about the same number of flights out of a charge as I did before with the added benefit of the faster servo response. I also use Voltwatch and monitor my battery voltage before AND after each flight. But, being a cautious soul, I don't let the battery go down to the point where I would have any question of making it back to the runway. If it looks as though it needs to be charged, I charge it. Call me over-cautious, but I've never lost a plane because of a dead battery pack because I flew just one-more-time.
I've read some posts here where people fly when it is clear that their battery pack is just about depleted. That's taking one heck of a chance. Not only with crashing your plane, but where it may come down. Remember Murphy's Law.. and Murphy is alive and well.
CGr