I don't use my 1200GS off-road so I reckon the sensible verdict for me would be the 1200R.
There are some minor points in favour of the GS - handguards for one are a boon in the wet, and the ride height means I can bump up and down kerbs with impunity. The real difference for me is the riding position - I love the "big" feel of the GS and the fact that you can look over lots of traffic.
However, as and when the time comes to replace my GS, I think I'll take a longer spin on a 1200R.
Let us know what you decide.