There are a couple of differences between the two, a Tom Collins is made with "Old Tom Gin", a sweetened gin similar to Genever and is properly served in a ten ounce glass as opposed to an eight ounce glass for the Gin Fizz. Though hard to find Old Tom Gin is currently being produced by Hayman's.
Of course, you don't have to worry about that with my Gin Fizz Martini because I'm totally messing with the recipe by reducing the soda to just a splash so the cocktail will fit in a martini glass!
Enjoy and have a wonderful weekend!
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