The GSA1150 is in my opinion a different motorcycle to the GS1150. I owned a GS1150 for 5 years then bought a GSA1150 almost 1 and a half years ago now.
The GSA1150 is heavier, more planted, feels more "weighty" around the middle and is supremely more comfortable, better specced, has superior suspension and feels superb on the road. I loved my GS1150 and still rate it highly but the GSA is in so many respects a better motorcycle.
I have ridden many motorcycles since, including the really superb GSA1200, but I always come back to the GSA1150 and its unique riding experience.
I would really like to own both a GSA1200 and keep my GSA1150, but that would require some forward planning.
Back on topic, I'd suggest you test ride a GSA1150 for an hour and see for yourself. A nice one is a pleasure to own in ways I can't really describe.
Good luck whatever you decide.