Looks? Subjective I know but for me they both look more purposeful and like a bike that can do everything in the real world rather than sit in a wind tunnel. It's a motorcycle after all.
Comfort? More leg room, less lean forward and the ability to stand up during longer stints in the seat.
Luggage capacity? Nothing in it really and I've ended up carrying an RT riders waterproofs in my add on luggage system because they wouldn't fit in the non expandable luggage system of the RT. I know you can get expandable racks for the RT but where as the GS pulls these off, The RT looks like a Sierra with a roof rack!
Weather protection? Fuck off! Most RT riders don't go out in the rain
Saying that, I can do 125mph on my GS with a vario screen and my visor stays open, if I struggle to get them speeds on my RT my visor slams shut.
Weight? The GS is lighter and noticeably when manoeuvring around out of the garage.
Weight again? The GS is far more nimble when pushing through the twisty roads.
Riding position? You're sat in the bike on a GS with a more commanding view of the road whereas on the RT your sat on it with less of a view.
Gearing? The GS is suited more for 'every day' riding whereas the RT is suited to motorway mile crunching.
Ps. I've never rode a GSA? But I believe it has a bigger frontal area for weather protection than an RT and a bigger tank range too