What's the difference between a Zigbee hub and a Z-Wave hub?

Zigbee and Z-Wave are two separate low-power wireless protocols, and a device built for one won't talk to the other. Many modern hubs (like the Aeotec/SmartThings and Hubitat models here) include radios for both, plus increasingly Matter and Thread, so a multi-protocol hub is the safest bet if you haven't committed to a device ecosystem yet.