The life of each and every sprocket is mainly determined by the care and maintenance of your chain.
All sprockets, no matter what material they are made of, will become deformed in time when they are run with a stretched or faulty chain.
Aluminum sprockets will show this wear quicker than steel will, however steel sprockets are more robust and will tend to help kill your chain faster than it would with the softer sprocket.
If you keep an eye on your chain and don't let it elongate past 3% of it's stock length, ANY sprocket will do. But like I said, the steel ones tend to fight the change that is inevitible and they will trasfer the force back to the chain links themselves. An aluminum sprocket will tend to let the teeth deform quicker. In either instance it's the chains doings, not the material of the sprocket that counts.
Next post will be something like "I run steel sprockets and they last forever" well....I can make an aluminum sprocket last just as long as the steel. All wagers accepted! :)