I knew you were going to ask that eventually
. I'l review each one individually and you be the judge. Mind you, I haven't owned the GS-1 so take my words with a grain of salt on it.
I thought the concerto had a very clean, highly transparent sound. The mids were cleanly rendered and had good body while being very transparent and open. Detail retrieval was excellent with a wide soundstage. Listening to the guitar in Eric Clapton's Tears In Heaven brought me to my knees when I first heard it through this amp. Bass was equally impressive with good weight, control and extension. The crossfeed is a nice feature to have which gives you an "out-of-head" experience.
Coming over to the GS-1, the bass is big, fast, linear and well controlled. There is no smear or overhang but no leaness either. The midrange is definitely one of the amps highlights. They were clean and clear with a sense of body and richness similar to a tube amp. The mids seemed to have presence but not "in your face" sound. The sound was nicely detailed but not hyper detailed. Soundstage was big and open but not as big as the concerto.
I don't think you would go wrong with either amp.
I got to hear someone's Gilmore Lite with the powersupply at a recent meet and it sounded identical to the GS-1. You may want to look into that as well, would save you some money.