7.2 volts vs 14.4

[uarghhh]

hi guys... Got a big question that need a big answer :) ok... can someone explain me the difference between these 2 system :

1 - 7.2 volts, 5000kv = 36 000 rpm.... (like novak system)

2- 14.4 volts, 2500 kv = 36 000 rpm... (like 8xl or something like that)

what's the big deal? why you guys just pray for high voltage?

thanks in advance :)