Garry Regan claims that the original version is made with gin, but acknowledges the cognac variation. Cognac seems more, well, French, but I've heard that the drink was named in New Orleans after a type of military weaponry, so perhaps the Frenchiness of cognac is not evidence that the cocktail should be made with cognac.
Puzzled, I turned to Dale DeGroff. To my surprise, Degroff's answer is: none of the above! Well, sort of. He makes his French 75 with brandy. Now, I do realize that cognac is brandy, but I've never seen a cocktail list with a French 75 that contained any sort of brandy other than cognac.
(Apparently, there is also a lemon/lime debate with respect to this cocktail. It's not much of a decision for me: I'm a lemon man.)
So I put the question to you: When you order a French 75, what do you expect it to contain?