I choose the D700 recently OVER the D600. My personal experience was not good - as the models I trialed were dust magnets, having visible dust spots after as little as 300 shots.
This is apparently not an uncommon issue and you can check out the d600 forums here for more info.
I was coming from a D7000 so the control layout of the D600 was very familiar to me, although I am finding the ergonomics of the d700 more to my liking. I really wanted to like the D600 as it has slightly better dynamic range than the D700 but I seriously couldn't fathom the continuous cleaning needed for a brand new camera.
I personally think the extra MPs can be as much a curse as a blessing as it will make camera shake more evident in photos. I am yet to encounter a photo where 12 MPs wasn't enough to get the job done. But I don't print bigger than 13x19. However, if the D600 didn't attract dust like a ceiling fan I wouldn't have been deterred by the extra MPs.