I have both a 1250 GSA and a 850GSA (1250 and 850 from here on) to my free disposal.
Even if the 1250 is the more powerful, I prefer the 850 when ever I ride solo. Yes. the 1250 has better acceleration and engine braking, but for all practical purpose, it does not matter.
The 1250 is the preferred bike when riding with the MS, and riding with some of my buddies with 1250 that likes to keep a frisk pace. Even if I'm able to keep their pace with the 850, it takes a bit of an effort.
I like the 850 for it's lower weight, easier to ride and easier to handle with engine off. Even if the 850 is not considered a light bike compared to other bikes in this range, it still feels lighter compared to the 1250.. I also find the Quick Shift to be working a lot smoother on the 850 compared to the 1250.
My preference for the GSA over the GS was due to improved range and weather protection. Also, selecting a GSA made it possible to share the boxes with the 1250 GSA.
Most people don't think much of the improved range over the GS, but frankly, when riding with buddies with the lower range GS, riding with areas where there is some distance between the gas stations means that there is one less thing to worry about.