As a general rule, the thicker the branch, the thicker the wire it will require to hold it in place. Copper wire is stronger than aluminum wire of the same diameter. As a result, aluminum can be easier to work with, but you may need a thicker gauge wire to do the equivalent work that a lesser guage copper wire could accomplish. One advantage of a thicker gauge wire is that it has more surface area to distribute the pressure of the wire onto the bark with so it may bite into the trunk less than a thinner wire would. For bonsai wiring, the thinnest usable aluminum gauge would be 1mm and .5mm would be the lowest recommended copper gauge. Often times it is better to wire a branch using two wires in parallel than use a heavier gauge wire. More training and wiring information.