The three designs I know of are shaftless- only Tunze. Ceramic shaft- most modern quality manufacturers, and titanium or stainless steel. Stainless Steel being the mainstay of economy models. I know of no plastic shaft- perhaps you mean ceramic which in the case of Eheim and many others is white, lower quality ceramics have a brown appearance. Ceramic is superior in durability in many ways, it is harder but more brittle so breaks easily if mishandled. Shaftless of course eliminates a major source of friction but adds to design complexity and the number of parts. Their are also designs from Italy that mave the magnet fused to the shaft and rotating on teflon axels. In all honesty so much of this falls into gimmickry and the useful information that results from the design can ge gathered by safety underwritings and energy effficiency. All designs have their strengths and weaknesses outside of that. Stainless Steel can and will corrode but this is not going to harm an aquarium- Iron Oxide is relatively insoluble and what little bit does diissolve will cause no trouble.
As for the magnets, most magnets of a modern impeller are composed of ceramic or rare earth elements and thus don't corrode or readily deteriorate. I can only think of one brand no longer made that used vinyl coated iron magnets, this design didn't work very well and fell apart over time.
Admittedly their are at least a dozen details I could have added but then you end up with an overwhelming body of data. I decided that rather than going into the safety features of every pump- grounded, double insulated, thermastatic protection, etc. That it was far simpler to say- VDE or UL is willing to accept liability if the product causes a safety hazard in proper use. I see the design of the impellers the same way. Their are literally doens of features that contribute to efficiency and I could have included manufacturing tolerances, internal operating temps, shaft design, magnet type, unidirectional impeller styles, impeller well material (polyamide, nylon, abs, proprietary blend) these all have a large impact but the cumulative impact and overall thought and quality of engineering is generally revealed by the Efficiency Index. Further, most manufacturers would not be too pleased to have the details of their product released to the public, they would prefer that the competitors have to buy one and struggle on their own to find out what makes their product unique and how to copy it with out infringing on patents. I tried to work in cooperation with the manufacturers as much as possible in gathering this data and I did spot checks on my own instruments to make sure the data was within reasonable accuracy. If you have a specific pump question I would be happy to answer it by PM.
