Arminius,
The motors, ESCs, and accessories are primarily what determines your supply voltage. If you are using 6 volt motors, there's not really much point in using 12 volt batteries. Or, if you only have access to 12 volt motors, and ESCs, a supply voltage of 12 volts makes sense (you can series a couple of 6 volt batteries). Another consideration is the amount of volume and weight your model can handle. I seriously doubt if the 'Aziz' would have volume or weight restrictions that will limit the size of battery used (within reason!), so '6' or '12' volts is up to you and what you have available. There's no overwhelming reason for either voltage...
- 'Doc