I guess I will jump into this opinion pool. I have had 2 Sharks and 2 SG's. Alot of this comes to down to a couple of factors, which all are already mentioned. First is your body type and size (width and height). your body acts as a sail and traps air and circulates air as well. Next is shield. For folks that have ridden bikes with stock shields or shields that are too low, then you are not getting the best riding experience possible. One of the first things we do when we get a new ride is change the shield out.
Now, as far as ride quality, the RG (for me) was a little smoother at hiway speeds, lets say 70+, and yes more stable while passing the 18 wheeler. But i say a little. enough to sway my purchase, no. At slow speeds, less than 30, it felt a little heavy to me. The SG I have (had a 14, now a 15) rides great. I changed out the shield and I have no buffeting at all. Yes, the air comes up from lowers but it doesn't impede my ride or my grin. If I were to do it again, I would go SG. great ride and almost as smooth as the RG, but with better looks and positioning, for me.
So, for me it comes down to looks. I like the looks of the SG better, I also like that I can reach the controls, if and when I want to. Peace all.