> I also have a D7000 and I am trying to decide if I want to add the D7100 or the D600.
My advice: make a list of your priorities. See if the either of the cameras meets all of them. Choose the less expensive of the ones that meets all of them. One of those priorities might be to keep total system cost within some budget $X.
> I just want good image quality.
They both deliver EXCELLENT image quality. The differences are in the margins, and those margins are diminishing with each generation. FX has an inherent advantage in that for a given pixel density it necessarily uses large photosites. For example, both the D300 and the D3 offer 12mp output; since the D3 is FX and thus 848 sq mm vs 345 sq mm, the photosites on the D3 are larger. Similarly for the D7100 and D600. But that advantage is only manifested when the photosite size makes a difference, ie at high ISO. Don't shoot at high ISO? It won't make much if any difference. As I noted above, I often shoot with a DX and FX of similar output size (12mp, D2x and D3). I import those files into a single directory with LR4, and since of course I cannot literally shoot them at the same time they are sorted by time and are interleaved on the Library display. I normally have to literally look at the metadata to see which camera took which frame. (All of the lenses I would use in this context are FX, for example 24-70, 70-200, 400/f2.8 and TC-20eIII, so that doesn't give it away at a glance.) I'm traveling and don't have my archives with me or I'd post a few.
There are other differences, but they're NOT down to FX or DX. The D3 is laughably better AF than most of my DX cameras - but that's because it's a D3 with CAM3500, the others are old, perhaps much older and equipped with less capable systems. The D7100 and D600 are pretty close in capability here, unlike my inventory. For example, fast burst rates are also not something that is format dependent; from a technical point of view, 24mp is 24mp and getting those data cleared out of the buffer to the card is the same problem regardless of format. Perhaps getting the mirror out of the way at high rates is a little different - but that would be because the DX format has a physically smaller mirror that might be a tad easier to damp, and the DX frame is physically smaller so can be recycled slightly more quickly. It's not an inherent FX advantage at all! The fact that the D3 and D4 run at higher frame rates is actually a greater challenge to Nikon's engineers in FX than DX.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!