These comments are mainly about 35mm film.
Plastic flexes more.
Stainless steel can go out of alignment if you bang it.
I have great difficulty using plastic reels, because they require you to use two hands in a coordinated manner that keeps the sides of the reels parallel. For most people, that is easy, but not for me. I have one hand that is much less dexterious than the other.
It took a while for me to learn to use the stainless steel reels, but once I adapted my technique to the reels, it became easy for me to use them. Because they are inherently more stiff, they suit my needs.
For 120, neither work well for me. I have the same problems with the plastic reels with 120 as I do with 35mm. With the steel reels, I have difficulty with the initial attachment of the film.
There are other brands I haven't tried yet (JOBO, for one, and Kindermann for another) that may make a difference. In the meantime, I've found that the old developing aprons are functional, especially when used in tanks designed for steel reels.
I'd suggest trying both, with practice film. I think that if you find that steel reels work for you, they are more convenient, but others might disagree

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Have fun, I know I do, and have done so, for almost 40 years.
Matt