Aluminum is cheaper and lighter, but it also has higher resistance for a given guage, and is more difficult to connect to. If you are an expert in such things, or know one, and need the advantages that aluminum gives, go ahead. If not, why not use the best conductor, copper? (Silver is slightly better, but it is cheaper to use a larger diameter copper). Make sure to use good insulation, 90°C rated or better. Also, running two sets of parallel wires instead of one can cut down on wire heating due to more surface area. Make sure to follow all applicable electrical codes. Inverters/UPS must be grounded properly, and treated with respect, since they put out potentially lethal voltage.