The better hardware isn't that big a deal. The negligible difference in processor speed won't matter much in the long run. It really depends on what's important to you. Reportedly, the SIII has pretty good battery life, so that may be a factor. However, if software is important, GNex all the way. First of all, we'll see an official Jelly Bean update LOOOONG before the GSIII will (pretty much whenever Verizon approves it, whereas Samsung actually has to overlay TouchWiz, which takes a while).
Also, the dev community is and always will be more robust, since it's a Nexus device, and the Verizon GSIII has a locked bootloader, meaning it will be more difficult for devs to develop for it, whereas rooting and unlocking the Nexus is a piece of cake.
Although people whine about the Gnex's camera, it's actually not half bad. I generally get decent photos out of it, and quite frankly, as I've always said, if you need high quality photos for something, get an SLR, don't depend on your cell phone camera. Also, some people were saddled with defective phones, but if you get one of those, just trade it in for another and keep in mind that not all of them are defective.