Well, it looks like I'm buying the GS. Roade it about 90 miles (all pavement) yesterday and I really liked the bike. The boxer engine is WAY different than the ST's V4...something of an acquired taste. But after an hour into the ride I began to "get it". Since it was a cool day (mid 40's) and I had no electric gear on, I really noticed how much cooler the bike is, so that will take some adjusting. The heated grips on the GS range from comfy (on low) to scalding (on high)...much more effective than the aftermarket heaters on the ST. The GS was somewhat noisier and windier than the ST...but not so much that I really objected. Still, I'd seriously consider a bigger windshield for cold weather rides.
Already planning some farkles...starting with a fuseblock. Probably share my Zumo between the ST and the GS. Need to add a power outlet for the Gerbing. The bike comes with a Sargent saddle...I'll give that a shot...early reaction was "not bad".
I will use the GS for riding secondary and fire roads in north GA mountains. Remains to be seen what I'll do with the ST, but for now, I'll own two bikes.