Metal is harder, lasts longer, and can ding/scratch easier than most plastics. If you ding it, you will hear a loud ding.
Plastic is hard, but not as hard. Som finishes can get peeled off if you’re not careful. Can chip/dent as metal, but easier. If you ding it, you hear a thunk.
Metal does usually sleep longer. Most YoYoJam yoyos use metal rims, like the Dark Magic, New Breed, Black Knight, and more. Metal is heavier and weighs it down more to keep it going.
Plastic can sleep, but mostly not as long because theres nothing to weigh it down.
Just remember for the sleep times, its how you throw it, not depending mostly on the yoyo.
Metal: Usually the yoyos like YoYoJam are started off with full plastic, then fitted with metal rims which make it heavier. About, a guess, 10 grams.
It mostly depends on which yoyo. Derlin plasic can be a lot heavier than YoYoJam plastic. (Mabye) All are different. If a yoyo is full plastic, or a molding cover, being hollow inside.