I would agree with the posts above for all but the most simple installations, where only one panel is used, or if you have specific use for power of a specific voltage like powering pre-existing 12 Volt devices, or 24V motors ect., I would use the highest voltage for which components are commonly available, which is 48V. The reason is that the amount of power moved through a circuit is equal to the product of the voltage and the current. The power lost in transmitting the energy over a wire is linearly related to the current only, and is independent of the voltage.
In other words, Voltage Drop = Current multiplied by resistance, and Power = Voltage Drop Multiplied by Current. The voltage drop and power lost is independent of what the starting voltage is so long as the starting voltage is sufficient that the output voltage would still be positive. The resistance is generally a linear function of the area of the conductor for wires. Also, for other components, such as diodes and rectifiers, the cost is usually much less for components rated for lower amperages.
If you were to use a load or a diversion controller such as a Trace C-40 and move 30 amps through it, for the same cost of the device, you would be able to move 360 Watts through it at 12 Volts and 1440 Watts through it at 48 volts. Hope this helps, Rich hagen