There are a couple of differences between the GS and GSA. Most notably is the large gas tank on the GSA and the taller suspension.
Beyond that, they are very much the same. The GSA comes with extra protection bars (and they’re priced into the bike), etc. but folks typically add them if they are going to go off-r0ad.
The larger tank on the GSA provides better wind protection and will put a bit more weight on the bike, especially if it’s full. It’s nice on longer trips, a bit much if you’re going off-road.
Hard cases are great to have, but again, if you’re going off-road – the trend is to use soft bags as they’re less likely to hurt an ankle in a tip-over or to catch your foot paddling through a rut.
Vario bags generally do not survive a tip-over too well. Aluminum and soft bags are much more durable. They’re great if you’re not planning on taking it off-road.
For the ADV rides the club does on dirt roads, any configuration of bike will work well. The off-road and dual-sport rides is where you’d worry about the type of bags, cylinder protection, skidplates, etc. – but very few people are really interested in taking these bikes into that environment (deep Michigan sand, etc.).
I’m a 1200 fan, so I’ll leave the commentary on the differences between the 800/850 and the 1200/1250 to others. (But I’d get a 1200…)
So, it all comes down to what you want to use the bike for. They’re all great!