Originally Posted by
tclune
There are a few things that are clearly superior about the 7x00 series over the 5x00 series. You need to decide for yourself whether they are worth the extra money to you or not.
First, the 7x00 series can micro-adjust focus on lenses. Especially for large-apertu8re lenses, like portrait lenses, this can be important. If you have a lens that doesn't quite focus precisely on a 5x00 series, you can work around that by using live mode, but it is a bit of a pain.
Second, the 7x00 series supports the full commander mode flash system, including off-camera flash control. The 5x00 does not include the off-camera support in the camera. The commander mode has a lot to recommend it, with just one big flaw -- it is IR control. IR requires line-of-sight to work, and may be blocked if, for example, you are using flash with a light box. RF is much more reliable, can work over a longer distance, and does not require line-of-sight. But, if that isn't a major concern, the Nikon CLS system is very good.
Third, the 7x00 series has true mirror lock-up (MLU). The 5x00 does not support MLU, although it does have a mode that inserts a one-second delay after raising the mirror before clicking the shutter. But one second is not enough to dissipate mirror slap in many contexts. MLU is essential for maximum control with macro photography unless you are using live view -- in which case it is irrelevant.
Fourth, the 7x00 has fully lossless raw image modes. The 5x00 series compresses the raw data by combining similar values into a single value. Toward the maximum brightness, 5x00 lumps five values into one. Instead of saving 4096 distinct values, it combines the brighter ones into increasingly-larger lumps to yield a total of around 1000 distinct values (I forgot the precise number of values). This can lead to posterization in some cases. It also makes exposure to the right (ETTR) downright perilous -- you are making major use of the most decimated region of the exposure values on that camera. I should mention that the 7x00 can use the decimated tables for saving data just like the 5x00 series does -- but it doesn't have to do so.
There are a couple of features that are well-regarded on the 7x00 that are worth mentioning. First, it has a pentaprism instead of a pentamirror in its viewfinder, which is both brighter and larger than the alternative. And second, the 7x00 has dual controls, which make the camera adjustments faster once you get used to them.
The 5x00 has an articulated viewing screen that some people find handy, especially for taking photos at odd angles -- down low or up high in a crowd.
FWIW
ETA: I forgot to mention that the 5x00 series does not support high-speed synch, while the 7x00 does. That means that the 7x00 can take photos using fill flash in bright sun. For the 5x00, you need to use a neutral density filter to lower the shutter speed to 1/200 in order to use fill flash in bright sun.