I've ridden an R1100GS off road and would say it had two issues. On proper slimy mud it really didn't want to play even with TCK-80's. On anything else it was fine until it got out of shape, then you simply had to pick the thing up there were no second chances. It's big enough to get to be a pain. I wouldn't fancy that seat height and weight two up, but I'm not build to ride PD replicas either. If you go for the big GS find some sand and mud and practice as much as you can.

I've ridden an F650 two up and the power was adequate but not stunning. The 650's will get anywhere even two up, you just end up using all the performance a lot of the time. Seats on trail bikes were invented by the Spanish inquisition or someone, so expect your passenger to ask for a replacement!

I rode my Bonneville without the sidecar, two up, up trails that would worry a few green laners. Snow doesn't stop it either, but you'd wonder just how much of this it will take before a sub-30 mph average gets boring. On the road it's nicer to ride than either BM and between the two in performance. Naked road bikes with rims that'll take a semi-knobley are cheaper than BM's.

Can you live with half a ton of rather tall BMW?
Can you live with very little luggage and a 60 mph tarmac cruising speed?
Can you live with something that might be down to walking pace off road?
Can you get your passenger to ride another bike?

I'd take the 650 two-up anywhere I knew I'd need to do over 100 miles off road, otherwise the road bike. For a big trip I'd want to know the passenger really could live without heated rollers and a spare ball gown, or they were carrying there own on there own bike. The compromise is the difficult choice, you need to pick which bad bits you can live with. Personally I fitted the sidecar and solved the whole luggage/weight/stability/dog/heated rollers thing, but that's just silly!

Andy