I rented a D600 for a week to see for myself if I wanted to actually buy one. I was definitely convinced of higher image quality in the D600. What I am seeing is greater tonal depth -- images from the D7000 seem slightly flatter and thinner if those descriptions make any sense to you. And in my non-scientific comparisons between the two bodies using identical lens, the D600 was the winner. I made prints from crops and my wife was able to pick the D600 versions easily. Her view is the the D600 images look "more real". I agree and I picked up my new camera with a 24-70 f/2.8 lens today.
PS- I might be wrong but I don't hear any additional mirror slap in the D600 and I actually think the Quiet Mode might be quieter than the D7000.