The truth
In reality there is not a great deal of difference, one from the other. I doubt if many buy the Adventure because of the extra fuel it can carry or the minor differences in gear ratios, etc;. There is no doubt in my mind that just as an 'ordinary' GS looks very impressive beside (say) a 1200R, the Adventure looks even more impressive against the basic model. That is why tmost of those who can afford it and are rich enough (and tall enough) buy the Adventure.