You can choose a 12-volt or 24-volt system based on the voltage of the loads you are trying to power.
In order to make the best decision, you will need to know what loads you want to power and what their voltage needs will be.
A system such as in an RV or one with other lower voltage appliances where most of your loads will operate on 12 volts, it will make sense to keep your system voltage at 12 volts. If you wired your system at 24 volts you would have to step down the voltage to power your 12-volt loads and this will result in a loss of efficiency.
If you are setting up a solar system to power 24-volt loads, or if you are going to primarily power A/C loads through an inverter, It will be beneficial to set up your system in 24-volts. When the system is wired at 24-volts you will have less resistance throughout your wiring and smaller gauge wires can be used. It is also more efficient to convert 24-volts to 120-volts (A/C) via an inverter.