Going with a 24V thruster allows someone to reduce their wiring requirements by half. Power = Current x Voltage. Therefore, if you have more voltage, you need less current to power something. The higher the current, the bigger the wire needed. This means that if you go with 12V you will need to use bigger wires than with 24V. Larger wire is more expensive and could pose installation difficulties, costing you more in labour as well. In your case this is NOT so important since you are installing a battery bank adjacent to the thrusters (which means you only need a short piece of wire). Just make sure your wiring (most likely to be 4/0) can accommodate the enormously large amp draw at 12 volts required to power the thruster.