Most spokes today are SS and non-magnetic so you should use the same type of material to get correct tension on all spokes. Another difference in spokes is diameter. Some spokes are the same diameter for the entire length from the "J" hook at the hub to the end of the threaded area at the nipples. Other spokes have one thickness at the ends, the hub or the rim, and have a thinner section between those points. This type are called butted spokes and their purpose in life is to provide the necessary thickness at the ends where needed and the thinner section, since it can stretch a bit more, will distribute tension more evenly to the rim which in turn produces greater durability of both rim and spokes. It is best to be consistent in spoke material for any one wheel. I hope tis helps.