GS 3:
- storage
- battery life
- audio
- camera overall
N4:
- build/perceived quality
- stock android so get updates fast
- CPU/GPU and 1GB more RAM
- cheap
Screen is personal preference, NLED is good on both and design/looks is personal preference as well as software skin (touchwiz VS stock)
Comes down to what is more important.
Battery life, audio and camera are 3 important areas for me and so far going by the reviews/first impressions the N4 doesn't seem to be as good thus I will be staying with my one S as the only areas where the N4 is better (for me) is the nexus part so get updates quicker (I prefer sense over stock android though), NLED and the screen (more so for the size as I love the quality of the SAMOLED screen on the S and only wish it was bigger). The GPU/CPU is very nice but I doubt there will be much if any difference between it and my S (when it gets JB) as android and the apps etc. don't properly make full use of dual core let alone quad core CPU and I have not noticed any slow down on my S in games etc. at all
If I had the Gs 3, the advantages of the N4 for me would be, design, build/perceived quality, stock android, hardware.