Just an opinion, but if you actually want to take the thing off road, I'd say get the F650. I've had one for about 2 years now to use as a "foolin' around" bike for rides when I'm more interested in the world around me than carving. High speeds are fun, but the world seems to narrow down like tunnel vision to the just the road ahead, and then there's that whole visit to the Judge thing. You miss alot that way. My 650 GS has had just under 8k of dirt roads, abandoned rail beds, farm roads, and my wife toured on it last year on our vacation to the Blue Ridge. It also handles pavement suprisingly well, too, although the long, soft suspension gives new meaning to the phrase "brake dive", and it's tall enough that you get suprised at first on corners. Your head gets a whole lot closer to roadsigns and oncoming SUV mirrors than you're used to accounting for.
I looked at an R1150GS the same day I bought the 650, and I quickly realised that while it isn't that hard to pick an R1150 GS up on nice level ground, I'd be screwed if I dropped it on a muddy slope. It's just too damn big and heavy. Yesterday I was on an abandoned railroad grade and a fallen tree pushed me too close to the offside of the gravel side slope. The gravel turned out to be a whole lot less firm than I thought, gave way, and the bike slid out and ended up on its' side with a small tree wedged between the shift lever and the motor on the down side of the incline. If I'd have had the big GS, I'd have been screwed. It'd still be there now, waiting for a crane.
While the new R1200 GS looks like a nice improvement over the R1150 GS, the F650 is a good enough bike on its' own that if you're going to go exploreing on anything more rugged than a groomed dirt road, you'll be pleased. Yes, your freinds will laugh at you (right, Fernando?) for being on such a small bike, but the first time you get it on a bad road you'll be laughing your ass off. Taking gravel roads at 60+ mph, and aiming for potholes and puddles is a hech of a lot of fun. Try that on yer 748. I sure wouldn't on my Daytona.
The Multistrada looks like a whole lot of fun on paved back roads, especially rough ones, but I have no experience with it, nor has anyone I know. It's 17" wheels aren't going to give you access to any god dual-sport tires, though.
So, I guess it depends on where and how you want to ride. Whichever way, good luck. If you go for the Multidtrada, give us a review, okay? Fernando and I are both real curious.