I also have the S3 and it is a stellar device. Previous phone was the Razr Maxx. Radios are equal with the two and I live in a fringe 4G area. I did upgrade the baseband (radio) in the S3 and now it is better than the Maxx. If you get the S3 this is something you will see in a OTA. The Maxx is a great phone and the Nexus is good also. Just data drops and signal issues with the Nexus, so I have heard. Anyway, my S3 lives it non use life in my back pocket most of the time and I am 6'1" 220 Lbs and there is not a mark or crack on it. This holds true for the phone it replaced also, the Maxx. I am superiorly happy with the Samsung. It is my first non-moto phone. Motorola has build quality down but being innovative and taking the next step to be a leader (which they lead the pack at one time) is falling short as of late. The S3 will indeed get jelly bean and also remember that as OS's get bigger and better, IMO, the S3 is a better way to go than the "current" Nexus with the processor being superior and 2 GB of ram.
This is just my random thoughts that I can add. Do with them what you will. It boils down to this for me: S3 is very fast and smooth and the screen is big and the features are never ending. Its a great device and you would be happy with it as well.
I have not noticed any flaws with email as the above poster stated......the gmail is awesome and the standard email app is push so you have to set up the push times for it to check for mail. Gmail though is instant