Whatever you think of modern-day Rose's Lime Cordial (other brands are available as well as homemade recipes), as with the Gimlet, I believe lime cordial...
I like Difford's old-timey idea of using both lime juice and cordial, but his recipe looks pretty darn sweet. I followed Jimmy's 4:1 ratio of gin:lime juice and cut back on the curacao (used a tad less than the lime juice) to accommodate an equal amount of cordial (sans sugar syrup), plus a couple lime twists in the shaker. This gave a touch of tartness to the drink and seemed about right for a sweltering, un-airconditioned afternoon at the Club in old Rangoon. And my cordial was homemade lemon-lime-kumquat, which made things interesting.