You say 'mostly off road' but that could mean lots of different things from gravel touring to full on bush bashing in the outback. I have not heard much about the frame strength of the 1100 or 1150gs. The Rally bikes have modified and braced frames, but that is serious off road! I would offer that, given the choices you are considering, the F650GS Dakar would be the bike to take. If you are not planning to carry a passenger, and want the off road capability, then the lighter bike is a good bet. Trade offs will be in high speed tarmac touring and fuel range (the 650's have an under seat fuel cell that it difficult to enlarge). Again, it depends on what your exact plans are! I would suggest contacting HPN
info@hpn.de in Germany about the frame strength issue. They have some experience in BMW frame mods and will likely know of the weaknesses in the design. Good luck anyhow.