I really liked the 2011 and 2012 GSAs that I have ridden. I would trade from an RT to a GSA myself.
To offer a thought on if it is comparable, over on BMWSPORTTOURING.com they are focused primarily on sport touring, obviously, bikes like the RT and GT models, but so many folks there have moved into the GS or GSA that they started GS specific page for people so they can stay on the site, but focus their questions and discussions on the GS style bikes.
Personally I prefer the feel of the GSA over the feel of the GS, but both are good. If you are looking for a GS style bike for the occasional fire road, the GS is moree than capable and the handling is a bit closer to the RT because it is a bit lower CG. The GSA being taller and a bit more top heavy will feel like a bigger difference from the RT, but I feel it is better for the dirt on ground clearance, suspension trave and things like that. The extra fuel and crash bars are nice on the GSA but the question is, do you really need that and does it warrant the extra up front expense?
Just go take some test rides and see what you think.
it is easier to ask for forgiveness than permission