Not too exciting of a story to tell, lol. It was definitely between the Gnex and the GS3. A few things played into my decision.
1) Future software. I know the SIII is getting fully JB support and probably whatever comes next (Key Lime Pie?). But the Gnex will have it in working form quickly and easily. And it will also have the L build of Android after that, and who knows how long it will ultimately be supported. Being a non-google device, I don't have that same assurance with the SIII.
2) The Gnex is a rooter's dream. There are like 6 kernels and a bunch of different roms to tinker around with. Granted you're running modified stock, but they all seem to work more or less with very few issues. Back on the old days of CM7, GPS was very difficult to get to work, there'd be other buggy stuff etc. And you never could depend on HTC to release source in a timely manner, so that always held back AOSP development as well.
3) $$$. It's only $50 on amazon right now. The S3 is three times the cost. Does the S3 have the best hardware on the market right now? Sure it does (either that or the LTEvo I guess). But that changes every 6 months. So again, it comes down to software support. I also managed to snag free extended batteries through Sprint for just the cost of shipping (about $13) since they're running a special for any Gnex owner right now. So now I'm going to get a stock battery with pretty much the same capacity as the S3.
Those are pretty much my biggest things. The only downside I can think of on the Gnex is the camera. I've heard the light sensor isn't as good, obviously it's only 5MP instead of 8. I guess if you can't live without the best camera available, then maybe you should think about the S3. Or if you really like being able to switch between Touchwiz/Sense and AOSP. I always prefer AOSP. So I went with the Gnex.