Remember the kV must be matched to the voltage! 6S isn't always quicker than 4S! If you use an appropriate motor on both, with correct gearing, they will be effectively the same, same torque, same top end etc. What 6S will have over 4S is that it is more efficient, and so you should get a touch more run-time, and the components will stay cooler.
The electrics do let it down though.
I wouldn't bother with the Speed Passion system. The ESC is the same as the one in the Caster! Speed Passion are a brand of Hobbywing, and the Hobbywing 80A ESC is the same as the Speed Passion Silver Arrow ESC (only the Silver Arrow has a bent piece of aluminium on it, and no fan). The MMM is far superior.
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