Aftermarket wheels come in a variety of styles and construction materials, but why? Is one better than the other? That really depends on your definition of "better." Most aftermarket wheels will dramatically up the ante of your bike's looks, and some (but not all) can shave as much as 10 pounds of unsprung weight compared to stock.
Good looks are obvious, but why is lighter better? A spinning wheel creates gyroscopic force that makes it dynamically stable, and the amount of weight needed for stability is not very much. With the tire and brake rotors included in the total weight of spinning material there is enough mass for stability even if the wheel is as light as a feather. By making the wheels lighter your bike will be easier to flick through your favorite chicane, accelerate faster and stop quicker. Also, your suspension will have less unsprung mass to deal with, making its job significantly less demanding.
Sure, aftermarket wheels look trick and save weight, but are all aftermarket wheels safe? Every wheel has to pass a strict international standard series of tests to be sold at retail. Testing is a tough process-the wheel is put through several punishing scenarios including a torsional load test (simulates starting and stopping), side load test (simulates the stress a wheel is subjected to during cornering), impact test (simulates hitting a large pothole) and a radial load test (simulates miles of bumpy road).