[BrianG]

Electrically speaking, you will get a certain amperage given a certain voltage and resistance (load). If you increase voltage, amperage increases also assuming the load stays the same. These statements work great for DC. However, these motors are essentially AC driven motors (the ESC converts battery DC into PWM squarish-wave AC), so it gets a little trickier. You have to factor in inductance and to a small degree capacitance , which creates current/voltage phase angles blah, blah,blah. And we can't forget back EMF. As Squee has eluded to, these motors will have the most back EMF at their ideal speed. This is sorta like added resistance, which brings the amperage down. Below their ideal speed, they won't be as efficient. Also, all of this depends on the motor's load (acceleration, vehicle weight, and terrain)

Long story short: higher voltage will increase current draw until the motor gets in its RPM "sweet spot". So if you plan on doing low speed runs for crawling or whatever, I'd go with lower voltage. You won't get the RPM sweet spot, but the lower voltage will help keep current down. If you plan on high speed runs, stick with high voltage. If you plan on doing high speed runs with a lot of standing starts or hard acceleration, you're gonna get heat, so it's up to us to find a way to get rid of it.