Horses for Courses it is....
I bought an RT1150 (single spark) new in Feb this year and traded it back in August after 13500kms for a GS Adventure (twin spark), on which I've done 6500kms. Here's my £0.2's worth...
On the motorway/dual carriageway the RT rules, top (220kmh)and cruising (170-190kmh) speeds much higher than the Adventure (200kmh/140-170kmh). Extra BHP and higher gearing can't be beaten.
On the really twisty stuff the GS Adventure has an edge it's much easier to move around on the bike and the GS can change direction a lot easier, making it more nimble when hustling through the mountains.
Rider comfort I've found to be similar but arrived at differently, the long seat on GS allows more movement while the RT seat is like a sofa, both benefit from a sheepskin seat pad though! For the pillion the RT is better but the sheepskin pad levels it up quite a bit (according to my wife anyway) but the grab handle is much better on the RT (according to my son!).
Wind and weather protection is fantastic on the RT, love the electric screen! On the Adventure, Sh*te! Tobinators and a big screen (CeeBaileys +7/+4) can give you about 85% upperbody but thats about as good as it gets for me.
City traffic.. the RT is narrower but I don't have any problems splitting lanes with a GS, the nimbleness of the GS does help at times, but the lower centre of gravity of the RT helped it as well... I rode with the RT seat on highest setting so don't really think I see anymore on the Adventure.
On lousy potholed roads and gravel tracks the RT loses out big time to the GS, it doesn't like them and makes itself bloody tiring to ride.. Forget about taking an RT off road!
Brakes, I liked the fully linked brakes on the RT, when I switched to the GS and used the foot brake I thought the servo was shagged! I'm used to it now, no problems with either set up after the first day or two.
Where the GS scores for me is that you can get about 90% of the RT's on road abilities (with a few mods) PLUS having rough/off road ability, which the RT just hasn't got.
If you loved your brother you'd weigh in a few quid and let him buy both!
