The difference, by the numbers
Both have Angostura bitters. Only the Lion’s Tail has bourbon, allspice dram, lime juice and simple syrup; only the Pegu Club has gin, orange curaçao, lime cordial and orange bitters. In parts, a Lion’s Tail is 4 whiskey, 1 allspice, 1 lime and ¼ syrup; a Pegu Club is 4 gin, 1 Curaçao and ¼ lime cordial. By volume, the Lion’s Tail is 63% spirit, 16% liqueur, 16% citrus and 5% syrup and sugar; the Pegu Club is 75% spirit, 19% liqueur and 6% syrup and sugar. That makes them close cousins on the ratio map.
The Pegu Club is the stronger drink in the glass (21.5% ABV against 18.3%), but being smaller it holds about the same alcohol (1.73 standard drinks each). The Lion’s Tail has more calories (about 204 against 188) and more sugar (7.8 g against 4.8 g). Both are shaken and served in a coupe.