My Mrs reckoned the GS was the best pillion bike, then again before that I had sportsbikes and since then just the Super Tenere which was a close second to the GS for comfort - but she preferred the smoother (Than Aircooled GS) Yamaha engine and its much smoother power delivery so it was a draw, my guess is the WC GS is a lot smoother.
Since then she got her own bike so her comments on the KTM are probably invalid as once you have ridden your own bike being on the back always sucks, but for what it is worth she reckons the GS was better.I think the Ergo seats are a must, I lasted about 3 weeks before going back to the dealer (The Mrs go on the back was pre Ergo seat)
If passenger comfort is your only buying criteria I would go for a Goldwing - or a car.
I suspect the latest GS will be a bit more comfortable for the passenger, but it is also dog slow and damned heavy.
My top tip is get her on her own bike and then you can ride whatever you bloody well want.