I prefer the D7K as a walkabout camera, as it offers very good performance with a light weight and small size. For an indoor setting, or more demanding outdoor situations, where I want better lenses, the D3X wins hands down. For night photography, a full frame sensor and fast lenses are the best, i.e the D3X. I don't like the exercise it gives my tired old body, but the results are often worth it.
Having said that, the D7K will give the D3X a run for its money for sure in most situations.
A D7K with a Sigma 18-200 and a Nikkor 35 mm prime is just about the most bang for the buck with excellent image quality presently on the market IMO.
What did you think the answer was going to be? 