I'm not sure about 5 cells but I know that the more voltage the more power given So 6 cells will offer better punch. Take care of your 4200's the same way as your 3700's except use a high amperage pulse discharger (20 amps +) after you are done using them. Then put a 10-20% charge into them. Then you equalize the cells before you charge them for your race.
Michael