It's not what the tank or even the reel is made of that distinguishs them but the method used to load them. Although I haven't seen any, you could theorectically make the two designs out of the other material however they probably wouldn't work very well for a variety of reasons or be very expensive (eg. plastic reel design made from metal!).
Steel reels load from the inside out, plastic from the outside in. Plastic work well and require less 'skill' IF they work as intended. As noted above, if damp or even humid (in a change bag) they can cause great stress as they bind the film and cause kinks in the emulsion (can't fix). Much easier to have a film loaded in a steel reel 'jump' tracks and touch, therefore preventing developer getting at the film or get your fingerprints all over it. Those who master steel reels swear by them (and at plastic), the rest of us buy a few extra plastic reels so we never have to try to load a wet one!
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