The choice of nipple material inspires zealotry — some prefer brass exclusively, some prefer aluminum, others a mix of materials. Brass nipples have several advantages over aluminum. Brass is cheaper, stronger, more corrosion resistant and has less friction from the threads. They’re available in silver and black. Aluminum nipples are superior in two regards. They do the same job at one third the weight and they’re available in a range of sharp colors. Finish-wise colors are not created equal: silver, black and lime are most consistent; green and gold are very good; and red and purple are good.
I recommend both materials and consider the usage context when making a decision. If you’re hard on gear, clean infrequently or ride in adverse conditions, then brass is the sensible choice. So for my daily driver and touring bikes, I ride brass nipples. For my road bikes, where performance and fashion are important, I choose aluminum. It’s a nice weight savings for small money.
Working with aluminum nipples shouldn’t inspire fear. A couple tips go a long way towards avoiding problems. First, use liberal lubrication. When building in the first place, make sure threads are fully covered with oil or other spoke preparation compound. When making adjustments down the road, dribble a thin oil (e.g. Triflow) into the nipple and into the rim at the base of the nipple. Second, it’s best to use a 4-sided spoke wrench to avoid deforming aluminum nipples. Compared to a regular 3-sided wrench, a better tool distributes forces more evenly. The Park Tool SW-40 is an example of a 4-sided spoke wrench that is a lightweight and inexpensive.
Find the right nipples for your wheels in the shop.