I copied the following from ...
http://www.mercurymarine.com/service-and-support/storage-and-maintenance/faqs/propellers/What are the differences between aluminum and stainless steel propellers?
Aluminum is by far the most popular material used today for recreational boat propellers. It is relatively low in cost, has good strength, good corrosion resistance, and is easily repaired. However, compared to stainless steel propellers, aluminum propellers are more easily nicked or bent. Stainless steel is the strongest, most durable of all materials used for propellers. In fact it is about five times stronger than aluminum. This is its greatest advantage. The strength of stainless steel propellers will help maintain engine performance because these propellers are more resistant to the small nicks and bends normally found on aluminum propellers, often after running for a short period of time.
Blades cast of stainless steel can be made thinner for better efficiency. Stainless steel is much more resistant to corrosion. Stainless props can be repaired easily, although more expensively than aluminum. They can cause some corrosion on nearby aluminum surfaces in saltwater if good anodic protection is not used. An aluminum propeller can usually be replaced with the nearest size and blade equivalent stainless steel propeller.
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FWIW ...
The section I highlighted is where SS has a performance advantage. They have thinner more efficient blades all the way down to the blade root. Aluminum props have to be thicker since A is weaker which means more mass churning through the water. More mass = less efficiency. Flexing of aluminium has nothing to do with performance and even if it did a proper design can accommodate for that.
Aluminum props can re-pitched and cupped.
As far hitting stuff the inner rubber hub is what helps prevent shock to the lower unit in either type of prop ... if they have one.
