Personally I would get the d600 and a new Nikkor 50mm 1.8 ( or better yet a 35mm) The step up in all respects is enormous. A lot can be said for concentrating on one prime lens. You current lens will work and still best the 3100. The D600 is a camera you will be happy with for 10 years. The recent 3-4 cycle was an infancy thing with digital... It has now surpassed analog... The next generation is not going to be as giant as previous. JD