High-speed on the straight slab? An RT all day long. Owned one as well.
The added heft of the GSA probably gives it an edge over the lighter GS when traveling at higher rates of speed.
The GS is nimble and I'd think more so than the GSA. I have a ton of fun up in the canyons with mine, the RT acted more like a car up there.
They (GS/A) are good all-around bikes, simple, proven, adaptable. Having said that, for 90mi. round trips, I'd choose the GS over the GSA unless the added features of the "bigger" bike are that important to you.
Have fun deciding,
2012 Vespa 300 Super
past: Ducatis, Aprilias, Moto Guzzis, a Husky, and some BMWs as well.