Though I agree with what your saying from a hardware perspective S3 hands down. I would love to put that S4 processor the screen and the camera into my GNex as I prefer it's aesthetics over the S3.
With the Nexus however its all about the software. I would argue my Nexus with mediocre hardware, running a modified vanilla Jelly Bean with a custom kernel over clocked to 1520, runs smoother & faster than the GS3 at the Verizon store. I might also add that everything on this rom works as it should, and I have little doubt that I will be tasting the "K" dessert as well.
So at least at this point in time I would say it is a matter of opinion.
Sent from my Liquid Smooth, Lean Kernel powered GNex, rocking JB, long before VZW ever even thinks of releasing it...