I'll throw my .02 in if only to reiterate the points above that struck home. The short answer is, I had an 1150adv... wrecked it and am buying another this week! They're that much fun!
Like John said, the GS brings home a certain "wow, I'm on a motorCYCLE again" feel. A good, visceral, organic bike feel that the plushness of the LT takes away.
The wind can be managed to your taste, from simply a Laminar lip to a full "popemobile" plexi fairing. Only you, with your own ergos can iron that out.
I do miss the cruise control on the long trips! Again, that can be addressed in the aftermarket.
Yes, your comfortable top cruising speed will be likely lower. Oh well, you'll adjust!
I did a couple 800+mi. days this summer. I think that possibly I was less tired on the GS than the LT because of the lower weight and lower CoG.
I would never describe the GS as a dirt bike. It is a "dirt road" bike. Even *some* jeep trail. Unless you're Jimmy Lewis or have SERIOUS offroad cred, it's not gonna be pretty in the rough stuff.
I will be heading straight to Rick Mayers place asap with the new bike! A seat definitly makes an LD warhorse outta this bike! Barbacks also but again, ergos are highly personal.
The 1150 can take either the 8g adv tank or the touratech 10g! You call it!
IIRC the 8.7g noted for the new 12GSA was a misprint. Real life, it's 7.3g I *think*.
For a pure LD machine, I'd like the shorter standard 1150GS w/ an oversize tank, popemobile screen and highway ottomans on the crashbars!
As always... YMMV!
Oh... BTW, that gravel road that disappears off to the left?
That's the road to the really cool campsites!