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Grind profile of the burr's. all grinder burrs are cut with a specific type of brewing in mind, some just wont go fine enough for espresso, others produce a very wide spectrum of grind sizes.
the baratza's use conical burs which are ostensibly multi-purpose, but actually tend towards being better for filter brewing than espresso. they will brew espresso, they just wont provide as good a result as others.
Great value for money, perfect for filter brewing, less good for espresso.