MCN just did a head-to-head between the GS and PanAm, and the results are pretty much the same as any big ADV comparison I've seen for the past few years: boy, the HD is a fantastic bike, but just not quite as refined and perfected as the GS.
They gave the GS five stars and the PanAm four. GS got thumbs up for it's refined package, flexible motor, road holding and excellence in all conditions. Thumbs down for the fully loaded price and ubiquity. The Pan America got credit for the strong engine, comfort, gadgets and uniqueness, and knocked for 'ponderous' steering, a few rough edges and 'sidestand action when the suspension lowers', but they don't clarify what that means. Oh, and they describe the GS as a luxurious Range Rover to the HD's more utilitarian Jeep...