I don’t think plastic hurts much less than metal. Any difference is unnoticeable. As Owen said, the roundness is what counts.
Mg is my favorite material. I like it because it is kinda’ brittle so when you drop it and cause burr’s you can just chip them off with your finger nail or like a key. I think it is the best material for 5A/freehand/counter weight play.
I don’t like plastic much they are just to darn easy to kill. drop one just wrong/right and it done. crack body or hub. not good plus they tend to have press in axle hubs that get worked out with lots of dropping.
I like the softer of the aluminum because it is easier to get rid of burr’s. I can normally just use the back of my knife or a brick wall. Where with the more rigid aluminum you really got too go at it with a file to smooth it out.
I don’t like Ti much because I have never dropped one that didn’t make a crazy loud noise like hurt your ears loud. Also they get sharp and are hard to work down with anything but a file.