It looks like the seats are longer than the 1200gs. If you look at the rear tail light on both bikes. If this is correct this would be great as I find the GS has less room than my 1150 adventure.
Kiwi if you look at the tech specs you'll find that the Adventure is 40mm longer than the basic bike probably down to the reworked rear chassis. If you look at the footpegs you'll see some difference there, but untill you can actually try one seat on the other it is difficult to tell where this 40 mm is to be found.
Looks like the tank is longer - hence sit marginally further back.
I would guess the extra length is down to the beak extension and rear rack, as the wheelbase of the Adventure is quoted as 9mm less than the standard GS (likely due to greater castor on the standard).