RE: 4.8V versus 6V for receiver battery?
Nice thing regarding 4.8v is all of your std chargers will work with them. I'm just moving up to my 2nd model (an UltraStick 40) after 3 years on our trainer and looked at 6.0v stuff but decided to stick with 4.8v.
NiCDs are also a bit more durable than NiMH but the latter typically comes in higher capacity. For a std 4 servo aircraft, a NiCD works just fine. It came time for me to replace the stock 600mAH NiCD in my trainer so I chose an 1100mAH NiCD pack (also 4.8v). My son and I flew 8 times the other week and I still had plenty of battery left. That was over 90 minutes of time in the air! We were getting tired and hungry - I don't see why you'd need more for a sport RC model.
Dave Thacker has some good advice on 4.8 vs. 6.0 and NiCD vs. NiMH on his Radical RC site.