if the metal ones go out of balance, ie slightly bent blade, then the vibration would wear out the water pump brg quicker due to the out of balance mass.
I have found that a 6 blade aluminium fan works quite well. These fans are designed for inline housings with electric motors and most have adjustable blades for pitch. They are available from most machinery places dealing with type of equipment.
Here in adelaide you can find plenty at Machinery Warehouse in wingfield

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