The aluminum ones wear faster, the steel ones are stronger. The aluminum ones have a lower rotating mass. If you are drag racing, and shooting for low weight setup then go with the aluminum ones. If you are looking for long life go with the steel. There are also some other metals they make sprockets out of that will last a long time and are light weight. Titanium, chromoly, etc. They are more expensive though.
Edit: I would stick with a reputable chain manufacturer.