Alright, here is a pro-Nexus review for you.
Jelly Bean is awesome. The new Voice Search and Google Now, spanks anything else out there (yes, S-Voice and Siri included). And you can have it today if you root and put a ROM on it. Project Butter makes it feel like I put a new processor in it, it's so much faster and smoother.
Regarding what timelord said, Battery life will not be any worse than on the SGS3, especially if you get the extended battery (which I think actually makes the phone more comfortable to hold). Plus the custom ROMs have very efficient kernels which help here too. The latest ROMs also include a volume bump.
As far as camera, stills may be average, but video is quite good IMO. Also there are some good apps to improve your shooting if you are interested.
Overall, if you ARE interested in rooting and installing custom ROMS, its Nexus by a healthy margin.
PS, I moved this into an existing thread comparing the two phones in our Device/Comparison forum. THanks for understanding