Originally Posted by Tomillia

In my opinion it depends on a few factors. Are you running 21.5 with an RPM limit? Then the sustained voltage of a bigger battery has not that much of an effect because the voltage doesn't influence the topspeed. And of course the track itself you are running on.
For me it is always a ~6000-6300 mah Battery at 230 - 270g, because i just don't like the heavy feeling of big batteries. The nimble feeling is worth nearly any other tradeoff.
But it requires that you have enough space to place weights on your chassis. An added advantage is that the weight is therefore placed lower.