Kodi 17 is still in beta. You have to apply for beta on Play Store. If you are satisfied with firmware based on Lollipop, then keep it like this. I moved to Nougat because of some memory leaks that made the TV box unusable after a while (I keep it always on). Since there are still updates for both firmwares, I would recommend to stay with Lollipop. Upgrade to the latest firmware, but back up everything beforehand.