have you stayed in both and what will be the big PLUS one over the other, besides pricing?
I've only stayed in the GN Carson Tower room several times. It is a nice enough standard hotel room. I have never stayed at the Circus Circus. The big PLUS is for the Golden Nugget is that when you leave the room, you are in arguably the nicest hotel Downtown, right on Fremont Street. Compare this to walking out of your room in Circus Circus and being in a hotel casino that caters to families and is generally considered run-down. Circus Circus is also at the very far end of the strip and a bit isolated. If this is your first trip to Vegas you may want to consider a mid-strip hotel rather than Circus Circus or Downtown. Best of luck!
By far - the location of GN makes it the preferred choice.