Everything is system dependent. I had au24's for quite some time, then replaced them with cardas GR. To me the biggest difference is the GR was more holographic, had better deliniation of instruments in space, and overall had more finesse. The AU24's a bit more opaque, but also warmer,. more forgiving. Both had escellent bass.
At the time, my all solid state system with fairly revealing speakers in my "live" listening room, the GR were also too agressive sounding. Gearge Cardas will tell you himself that unless EVERYTHING is near-perfect, dont get the GR, since it can/will expose other sins (in my case things were too harsh/bright - i actually would have preferred the AU24's in my system at that time). George would tell you he would recommend the golden cross in most "real world" systems, esp if there's any residual bite/glare/harshness.
Dont limit your choices to Cardas. Tell us more about your system, dude.