My impression of the two bikes based on long test rides in paved twisties is that the FGS is much easier to ride on road than my GSA and I imagine MUCH easier off road. The weight is the primary reason IMO.
These are machines and you need to use the right machine for the job at hand. Granted I'm a noob off road, I CANNOT imagine how difficult off road situations can be managed with my GSA. I know that I need to be 3x the rider I am to take the GSA beyond the fire roads. The FGS I can IMAGINE taking on a serious off road adventure, but IMO, for that sort of thing you'd be much better off with the KTM 990 or 690 (I've got the 690e for long offroad trips, I've also got a KTM EXC450 for DS day rides).
Even though I liked the FGS, I bought the GSA because it is the defacto round-the-world touring machine that will survive hell or high water, and it truly is pampering on the highway. That's not an reputation any other bike can claim, even an FGS. Now, if you're not going to use it for hard core touring with minor off roading, I see no reason to consider an RGS. If you plan on doing a lot of in town street and freeway commuting with lighter touring get the FGS, no question in my mind. IMHO, the FGS is much more versatile, practical and provides for what MOST people need from a light touring superenduro. Believe me (and flame me if you will), I even prefer the rotax motor over the bulky, vibrating boxer. It revs more freely and it's a more tidy, narrow package.
08 KTM 690E / 06 GSA / 07 450EXC / 05 Lotus Elise Lotus Cup Open Class
10 Audi A3 / '12 Land Rover LR4 / bicycles for every terrain
11 year old boy - baddest-ass toy I've ever had