You've done what I did many years ago,I went to my local Saab dealer to test drive a 9-3 having previously had a 900, the car was out on a test drive, but as I was looking at a 9-5 they offered me a drive in one of those "while I was waiting", which I did, then I drove the 9-3. Both cars had the same engine spec so the 9-3 was definitely faster, I bought a 9-5, not the one I drove, I wanted a blue one but the same spec, and a few years later I got a 9-5 Aero (the faster one).
I currently have an IS300h, cracking car, but I bet the GS will be even quieter and smoother as was the 9-5 vs 9-3, also the extra space inside would fell more luxurious too.
When I bought my IS, I too considered a GS, though my choice of spec and colour was out of my financial reach, I would say that if you prefer the GS go for it.
Vince