FCI recently performed an empirical comparison of shaking vs. stirring:

Shaking completely chills, dilutes and aerates a drink in around 15 seconds, after which the drink stops changing radically and reaches relative equilibrium. Shaking is basically insensitive to bartender-induced variables. …bartender skill is very important in a stirred cocktail. Because stirring doesn’t reach equilibrium, stirred drinks are warmer and less diluted than shaken cocktails. Stirred drinks, unlike shaken ones, are not aerated. Stirring does not alter the texture of a drink—it merely chills and dilutes. A properly diluted cocktail stored at -5 degrees Celsius in a freezer is indistinguishable from a properly stirred one. http://www.cookingissues.com/2010/09/02/cocktail-science-in-...