The standard system of measurement is a funny thing. Upon its founding, The USA adopted the British wine gallon (231 cubic inches), and derived the 16 ounce pint from that meaurement.
The British, however, switched to the 20 ounce imperial pint in 1824, derived from a new gallon measurement based on 10 pounds of distilled water.
What does this mean for beer drinkers? Four extra ounces of beer.
Many bars serve Guinness in an Imperial pint. Not so many serve all their beers in one. If you're looking for value, find a watering hole that pours all their beers into Imperial pints.