Many of the "features" on the SIII are quite disappointing. For one it comes preloaded with Samsung's version of Android, which is quite bogged down and eats your battery. The apps that Samsung loads on are almost all completely useless or rendered obsolete by Google's versions (ie. the Samsung Calendar (or S Calendar as they call it) is slower and has less features than the Google Calendar).
The Nexus 4 comes straight from Google, so you're not messing around with any third party uninstallable apps, as well as you're getting updates pushed straight from Google when they are released. That means that you won't be waiting for your carrier to approve or modify it before having it released.
All in all, the S3 has a nice, big screen, but the Nexus 4 is going to get better performance, fewer headaches and more support down the road.
I had the S3 and I'm just now moving towards the Nexus 4. Plus for the price, getting it straight from Google, shipped for less than $400 is pretty incredible.
