The C8 has an upgraded Zigbee chip which has helped somethings but made some other devices not play nice. Upgraded Zwave chip and external antennas for both which helps A LOT with range, and built in Wifi. Just about all the other hardware is the same.
For the C7 one of the things you will want to check is to update the zwave firmware (separate from hub firmware). This post explains how to do it: C7 2.3.1 and Z-Wave Firmware 7.17.1 . The post is a little dated and the current version is 7.18.1 but otherwise the update steps are the same.
