If you’ve not deployed them yet, then I would suggest getting them upgraded before you do that as it will save you time in the future.
Android 7 has better support for concurrent video playback (ie playing 2 videos at once), and we also plan to be able to offer incremental firmware upgrades without needing to touch the device, but you will need to be on Android 7 to start with for that to work. The Android 7 firmware also has a newer version of WebView, which may resolve some issues you may have with specific websites that trigger bugs in older Google WebView versions.
Finally Android 7 has support for LetsEncrypt X1 root certificate out the box, which may be a concern for you if you host your own CMS. There’s more details on that here:
Beyond that, and in the vast majority of scenarios, there should be little appreciable difference.