Just something to take into consideration here, if you use an internal mixer (i.e. through the tx/rx) it will limit the voltage to each motor by 50%. This means that your robot will be half as fast and powerful. However if you use an external mixer (i.e. one that is separate from the tx/rx) you wont encounter this problem.

I would also say that if this is your first robot, I can imagine that your just dying to get something moving. So for the time being either would probably be OK