I just upgraded from a D7100 to a D750. Both are great cameras, the main reason for the upgrade was that I do a lot of night time photography, astro etc., as well as some indoor stuff in dark places (auditoriums). The D7100 does a good job under these circumstances with a fast lens (just ask James, he’s seen what it can do), but the low light performance in the D750 is AMAZING. Large sensor=larger light receptors=more light gathering power. It’s also much less noisy at high ISO (you can get reasonable photos at 12800, which is impossibly high for a dx sensor).
If you were to go with the D750, I’d do a kit like this one https://www.bhphotovideo.com/c/product/1082604-REG/nikon_d750_dslr_camera_with.html with the 24-120mm f/4. It’s a great lense, and good combo for general shooting.
If you were to go DX, you can’t really go wrong with the D7100/7200. The sensors are outstanding, and even in low light they do a good job. FX is going to be better just because of physics, but I’ve shot tons of astro and event stuff in dark situations with my 7100 over the last 4 years, and it holds it’s own.
Incidentally I have a d7100 and a couple of lenses sitting on my desk looking for a buyer… 🙂
But, if you’ve got the cash – go with the D750, you won’t regret it at all.