If you've got the space for gearing, it's always better to go for low kv high voltage setups. Higher voltage is usually more efficient, and so are lower KV motors
At end the end of the day, with a higher voltage setup, you're gonna get more power. More speed requires more power, so again high voltage ftw.
On smaller cars gearing and physical space for the motor is the limiting factor.
Also bear in mind with a big low KV motor and lots of batteries you've got the added weight, and you need more space again.
Basically you need to work out a good balance of kv to voltage to weight.