I will tell you a quick story. I had a buddy who was a seasoned, professional photographer. He kind of took me under his wing for a minute before he moved away to Arizona to retire. We got together one day and I shot with his full frame Nikon DSLR and he shot with my D 3300, both had similar glass. At the end of the day we compared photos. I was shocked at the quality of his pics on my camera vs the ones I took with his. He told me that my camera had a sweet spot and I asked him where? He took his finger and tapped it on my head, inside there.
Two biggest differences between the 7100 and the 7000 would probably be the sensor and the AF system. The 7000 uses a 16 mp sensor, the 7100 goes to a 24 mp sensor. The higher MP sensors really come into play when you shoot telephoto more and need to crop images, you can crop a 24 mp image a lot more than a 16 mp sensor before it becomes noticeable. The AF system in the 7100 is a big step up from the one in the 7000, which again probably won't be too noticeable for portraits but makes a big difference when you start shooting action shots.
Probably the biggest difference between the 7200 and the 7100 is the buffer size, the 7200 allows you to shoot much longer bursts before the buffer fills and the camera has to slow it's FPS to compensate.