The biggest difference is the bike ergonomic...ie how you sit on the bike.
plug in your data and see the result.
simple said the 700GS sit more upright....not so much weight on the wrists...foot pegs more forward.
To me this is much more comfortable for long distance riding.
Clearly th 700GS has longer suspension travel...better for rough roads.
also there are tons and tons of farkles available for the 700/800gs.
When I desided I wanter to replace my R--RT with a lighter bike I looked seriously at the F800ST...took one for a 60 mile ride in the twisties....it is a great bike...but too much (for me) of a folded up, lots of weight on the wrists sitting position for long rides. I got on the F650GS and the ergos were perfect for me.
I added a good coverage wind screen (aeroflow) bags (jessie) and I've wandered around remote area quite a bit. Note 99.8% of my riding is on secondary and dirt roads...I seldom go truely off road ..a 500lb bike ..is just to heavy to do that again my opinion.
Love the 650GS