I've never ridden an 1150....but I am not sure I would want one on questionable terrain. They have a hundred or so pounds on the 650...25% more weight, right out of the box. I don't think I would like to pick one up all that often. Even my 650gs, loaded, is pushing 550 pounds, and is not fun to pick up. Plus, you just can't touch the gas milage of the 650gs (70mpg when I am nice to it, 60 when I am not).
But, I'll be honest, the 650 needs a bit of work, IMO. The suspension is very easy to overwealm when you load it down. The bar to peg distance is on the short side (puts your arms in a less than optimal position when standing up), the seat is downright painful, and there are a few choice pices of equipment missing....things like a chain guide.
But, those are all easily addressed (I said easy, not cheap). Read the summer issue of the Overland Journal for a more in-depth version of my opinions on the F650GS. Read the Winter issue for even more info....and watch for the wrap up next spring. They really are good bikes. They have a definate lean towards street performance, but they have a lot of things going for them when compared to the other adventure touring bikes....not the least of which is the fact that they are a BMW.
Oh, you might also look for the 2008 F650GS.....rumored to be showing up with a parallel twin instead of the single used through 07.
1999 Dodge: stock -- noisy, stinky & handles like a bloated pig.
2009 BMW F800GS: fast and sexy.
2000 Husaberg FE600E: Makes KTMs tremble with fear.
2007 Suzuki DR200: The 'girl' bike.
1981 Honda XL250S: olds cool funkiness.
1980 Honda CM400: rat-bike.
"Life is meant to be enjoyed, not endured" ~someone