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The D700 is a really great camera, I know I sure loved mine. But the D600 is definitely a step up, and the extra Dynamic Range, the better High ISO handling, sure will of a great benefit to you.
As to the dust/oil on the sensor with the D600, remember, from all accounts it's a temporary issue. It's one that goes away after about 3000 shots. And since Nikon has acknowledged the issue, I wonder if they have made adjustments in the assembly of the D600's now, so perhaps if you buy a new one, the dust is not still a current issue?
I will say that my using layer masks as I process my shots is cut down probably 75% with the D600 compared to amount of work I would I would typically do when I was processsing my D700 shots. I think for anyone, that in itself would make it worth the purchase.