As with almost anything, it comes down to personal preference. Having said that, here is my take on it.
Hardware - With the rumors of it being a dual core stateside, that means it won't be much better than the GN, if any better at all. The screen size realistically isn't that much different, 4.8" compared to 4.65". I would guess the camera would be an upgrade, but I am not much of a photographer.
Branding - The SGS2s in the US weren't much like the SGS2s in other parts of the world. But it is even different in the US, as carriers want differentiation between other carriers. So Samsung makes the phones different for each carrier. There have been ones with keyboards, ones with different sized screens, ones with slightly different internals....etc
Release date - When will the SGS3 come to Verizon? The SGS2 took a while to hit the US after the SGS2 release across the pond. Verizon tends to lag behind other carriers so, it could be a while.
Price - More than likely the SGS3 will be rather pricey when it comes out, the GN isn't too expensive anymore (depending on where you shop at least).
Updates - While I know the GN on Verizon is lagging behind on updates, Samsung devices in the US tend to be some of the worst devices about getting updates in a timely manner regardless of carrier.
Dev community (for rooters) - While I am sure the SGS3 will have a great community, the GN already has a strong one established.
I am not overly impressed with the SGS3, or any other new phone for that matter. No phone has wowed me away from the GN. Nothing they do my GN can't already do just as well, for my needs at least. Had I not already had the GN, and had the choice, I still may choose the GN just based on price per performance. Also I like the software softkeys, over the hardware softkeys. I am not saying that the GS3 is a bad device, but I am not completely sold on it, but then again I haven't held one in my hand yet either.