I bought my wife a Nexus 4... if I liked it, I planned on selling my S3 and getting another one for myself.
That way we could share accessories, car docks, etc... there would be some benefit to having the same exact phone.
Anyway... her Nexus 4 was delivered yesterday and I'm not sure that I like it any better than the S3.
The N4 seems bulkier and while the N4 does have a slightly higher resolution than the S3, some of that real estate is used for the soft buttons home, back and recent apps.
So when you have them side by side it looks like the S3's screen is bigger on a smaller device.
The play store is sold out of N4s right now anyway, so I don't really have a choice.
I think once CM10.1 (or any AOSP 4.2 based ROM) is stable there'd be no reason to go from the S3 to the N4 (until 4.3 comes out and you have that 3 to 4 month lag).
Also, my S3 has 32GB and a micro sd card slot.