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If you are a semi smoker or single, light, medium type smoker, a 40 MM metal grinder is probably your best bet.
Acrylic grinders are fine, they do the job, are cheap, and easy, but metal grinders hold up better long term, they tend to get a better grind, without missed chunks like acrylic grinders can sometime leave. Metal grinders also generally have a keef catcher, while most acrylics don't.
Unless you are grinding multiple grams a day, everyday, you generally won't need more than a 40MM grinder, but if you are grinding for 2 people, or are an active smoker who grinds alot of flower, nothing beats a solid 60 MM metal grinder, they generally last for years, have keef catchers, do the job quick, easy, and clean, regardless if it's 1/2 a gram or 2 1/2 grams at once.