Oh the choices...I was there myself several months ago. Test rode the RT and its a great bike, but the rugged, no-nonsense, get-me-dirty mentality of the GS/A is a breath of fresh air. I own a Harley; I didn't need a second bike that had
to be kept clean!!
That all being said....I'm 5'10" with a 33" inseam, so short in the torso, therefore sitting very well behind the windshield. In cold weather, I leave the shield up all the way...smooth sailing, no noise, no buffetting. Otherwise, I leave it one notch below full up. Same story...no buffetting.
EDIT: I wear a full face Shoei Qwest. If I open the visor, I still have good clean air, but get a whistle. In the proper down position, I don't get any air hitting the shield which in the cold weather means I have some fogging isues. And in the warm weather, well, it can be down right hot! I can drop the windshield to the the lowest or 2nd to lowest position and get more air to my face. This makes for a little more noise, but no buffetting.
My wife who is quite short - 5'2" and short in the leg has zero issues with wind and noise and plenty of legroom. We also have a Harley Street Glide, which buffetts the heck out of my full face helmet at anything above 60mph. She catches tons of air on the HD, not so much on the GSA.
The smooth air, smooth engine and incredible ride quality of the GSA is far superior to my Street Glide. The HD feels like an old bike in comparison...the GSA soaks up any road quality issues with no jarring into the rider and passenger.
Go spend some time on both the RT and GSA - take advantage of the liberal demo policies of BMW and take your time deciding. The GSA is so far my favorite ride....over the Harley, over my prior sportbikes. Its off-road capability adds to the adventures, but even if you don't plan on using it that way, its still a superior street machine than many other bikes given its incredible fuel range, long-travel suspension which soaks up anything the worst roads can give you.
Biggest thing I miss.......cruise control. The Harley totally spoiled me. I hate the cramp-buster thing so I guess a Kaoko is in my future.
Good luck on the purchasing journey! Make it fun and enjoy!!