I have metal, but have tended another's hives with plastic. I prefer the metal, for stability, and have found no difference in bees going through them, metal or plastic. With both, ya want the excluder right above the brood chamber, which urges them bees to go through with the surplus and store topside. (But I digress.)
Cleaning is an issue, eventually. With my metal, I can kinda slam them down when cold or actually frozen and that'll knock off enough for my satisfaction. (Or dip 'em in a vat of boiling water. Large crab cooking pot?)
With plastic, frozen requires special care to not flex it much as it is about as brittle as the wax stuck to it. And it can't be dipped in a large crab pot of boiling water, like a metal one. So finding that middle ground where the plastic is not breakable brittle, yet the wax on it is. At this point, the plastic excluders can be flexed "enough" for the bulk of the wax popped off.