Plastic/composite all the way.
You can not beat lightweight and technology advancement, hell even oil pans are now plastic . Its all because of the fact that plastic technology advances and Im very glad it does.
Cast metal WP are too heavy for the bearings and the bearing will eventually give out, much sooner then a plastic impeller will ever break. Before I put a Stewart in my car , my original plastic water pump had over 150k kms on it and still worked fine, on my other BMWs the plastic water pump impellers lasted 160 and 145 k kms. Good enough for me