People use aluminum pulleys in order to reduce the rotating mass. If the pulleys are made up of lightweight material, then it becomes really easy for them to easily accelerate and stop sooner, compared to the ones made up of heavyweight metals, such as steel and iron. Aluminum pulleys work best in this case, as they are lighter in weight and long-lasting compared to the nylon and plastic pulleys.
You should prefer forged aluminum pulleys over their cast version, as the manufacturing process of the forged aluminum pulley is much better than that of cast version.
Forged aluminum pulleys are durable compared to the cast aluminum pulleys; they have increased machining tolerances, toughness, and more diameter than cast versions.
Even though, aluminum is not prone to rust, the metal gets affected with corrosion process, if it is not well protected. You should always go for the aluminum pulleys which are anodized, in a bid to protect against corrosion.
You cannot ignore this factor, especially when the pulley will be used in or near water, or in wet atmosphere. Anodizing is a process in which the surface of aluminium is covered with a coloured protective layer. If you do not want the coloured coating, then the clear coating option is also available to you.
If you are unable to decide which brand to go for, then you can simply talk to people who use a particular brand of aluminum pulley and ask them about their experiences with the product. You can also look for product reviews online and compare different products before you make your final decision.