Larry: I wouldn’t worry about buyer’s remorse. I bought a used 2005 1200GS back in September, 2009 and have never regretted it. I don’t know if the new 1250’s are better/worse for fuel range, but my ’05 GS can do 180 miles per tank easily and I have stretched it to 220. The GSA would push this range up closer to 300. That is great for long trips, but largely worthless on a group ride unless you all have the big tanks. We typically joke and call the GSA’s the “Exxon Valdez” and the backup fuel supply for the smaller bikes.
Also, I know that my butt is usually ready for a break after 180 miles.
My comment on the sand was answered by Jeff H: it is all about the tires. The big GS is fine on any sandy Michigan trail with the right tires. Having said that – I would not recommend trying these trails alone. Any of these bikes are a chore to pickup yourself. That is what the club is for!
Also: I have all three sizes: 2018 310GS, 2003 F650GS and the ’05 1200GS – all setup for adventure rides. Hit me up with a message if you would like to take a spin on any of them.