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Cointreau is a Premium Orange Liqueur which is sold at one price; where as
Grand Marnier is a blend of oranges with Cocnac, and is sold in three price
ranges depending on the cognac used. My first choice is always the Cointreau.
With regard to the Premium designation, this usually depends on the age of the
product; the reputation of the Maker; and as you already observed, the price
is the first indicator.
There are tons of reviews on the WEB.